Null Error Exception on badge cart counter

Question

I'm trying to complete the functionality of shopping badge card. But, I face a problem When my shopping cart has many items, my app and the shopping badge works perfectly. Also, when I have 1 item in my shopping cart my app and the shopping badge works perfectly. But when i remove all items for my shopping cart and badge equals with 0, the next move will cause an error in the application. Αnd more specifically, it causes problem :

java.lang.NullPointerException: Attempt to read from field
   'android.widget.Button com.example.myapp.CartActivity.btnPlace' on a
   null object reference
at com.example.myapp.Adapter.CartAdapter.getItemCount(CartAdapter.java:120)
at com.example.myapp.FragmentDetails.CoffeeDetails.onOptionsItemSelected(CoffeeDetails.java:174)

Below I quote my code : My code of CartAdapter is:

public class CartAdapter extends RecyclerView.Adapter<CartAdapter.CartViewHolder> {
    
        private List<CoffeeOrder> listData = new ArrayList<>();
        CartActivity cart;
        private OnCartListener mOnCartListener;
    
    
        public  CartAdapter(CartActivity cart, List<CoffeeOrder> listData, OnCartListener onCartListener){
            this.listData = listData;
            this.cart = cart;
            this.mOnCartListener = onCartListener;
        }
    
    
        @NonNull
        @Override
        public CartViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
            View v = LayoutInflater.from(cart).inflate(R.layout.cart_item , parent, false);
            return new CartViewHolder(v,mOnCartListener);
        }
    
        @Override
        public void onBindViewHolder(@NonNull CartViewHolder holder, int position) {
    
            CoffeeOrder order = listData.get(position);
            holder.txt_cart_name.setText( order.getProductName());
            holder.txt_price.setText(order.getPrice() + " \u20ac");
    
    
            //change total price if quantity change from elegant number
            holder.img_cart_count.setNumber(listData.get(position).getQuantity());
            holder.img_cart_count.setOnValueChangeListener(new ElegantNumberButton.OnValueChangeListener() {
                @Override
                public void onValueChange(ElegantNumberButton view, int oldValue, int newValue) {
                    CoffeeOrder coffeeOrder = listData.get(position);
                    coffeeOrder.setQuantity(String.valueOf(newValue));
                    new Database(cart).updateCart(coffeeOrder);
    
    
                    //Update total price
                    //Calculate total price
                    Double double_total;
                    double_total = 0.00;
                    List <CoffeeOrder> coffeeOrders = new Database(cart).getCarts();
    
                    for(CoffeeOrder item:coffeeOrders) {
                        double_total += (Double.parseDouble(order.getPrice())) * (Double.parseDouble(item.getQuantity()));
                    }
    
                    String total = String.format("%.2f", double_total);
    
                    cart.txtTotalPrice.setText(total + " \u20ac");
    
    
                }
    
            });
    
    
            //remove item from shopping cart
            holder.itemDeleteBtn.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    CoffeeOrder coffeeOrder = listData.get(holder.getAdapterPosition());
                    listData.remove(holder.getAdapterPosition());
                    notifyItemRemoved(holder.getAdapterPosition());
                    new Database(cart).removeFromCart(coffeeOrder);
    
                    //Update total price
                    //Calculate total price
    
                    Double double_total;
                    double_total = 0.00;
                    List <CoffeeOrder> coffeeOrders = new Database(cart).getCarts();
                    for(CoffeeOrder item:coffeeOrders) {
                        double_total += (Double.parseDouble(order.getPrice())) * (Double.parseDouble(item.getQuantity()));
                    }
    
                    String total = String.format("%.2f", double_total);
    
                    cart.txtTotalPrice.setText(total + " \u20ac");
                }
            });
    
    
    
    
    
        }
    
        @Override
        public int getItemCount() {
    
            if (listData.size() == 0){
    
                //making it semi-transparent
    
                cart.btnPlace.setAlpha(.5f);
                cart.btnPlace.setEnabled(false);
                cart.empty_list.setVisibility(LinearLayout.VISIBLE);
            }
            return listData.size();
        }
    
        public static class CartViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
    
            public TextView txt_cart_name,txt_price;
            public ElegantNumberButton img_cart_count;
            public ImageView itemDeleteBtn;
            OnCartListener onCartListener;
    
    
    
            public CartViewHolder(View itemView, OnCartListener onCartListener){
                super(itemView);
                txt_cart_name = itemView.findViewById(R.id.cart_item_name);
                txt_price = itemView.findViewById(R.id.cart_item_price);
                img_cart_count = itemView.findViewById(R.id.btn_quantity);
                itemDeleteBtn = itemView.findViewById(R.id.btn_item_delete);
    
                this.onCartListener = onCartListener;
    
                txt_cart_name.setOnClickListener(this);
    
    
            }
    
            @Override
            public void onClick(View v) {
    
                onCartListener.onCartItemDelete(getAdapterPosition());
            }
    
    
        }
    
    
        public interface OnCartListener{
            void onCartItemDelete(int position);
        }
    
    
    }

My code of Activity is :

public class CoffeeDetails extends AppCompatActivity implements CartAdapter.OnCartListener{

    TextView coffee_name,coffee_price;
    ImageView coffee_image;
    CollapsingToolbarLayout collapsingToolbarLayout;
    Toolbar toolbar;

    String coffeeName="Coffee Name Not Set";
    String coffeePrice="Coffee Price Not Set";
    String coffeeImage="Coffee Image Not Set";

    Button btn_add_cart;

    FirebaseDatabase database;
    DatabaseReference coffee;

    String coffeeId = "";
    int ID;

    RadioGroup group_choose_varieties;
    RadioGroup group_choose_sugar;

    RadioButton rb2;
    RadioButton rb3;

    TextView text_buondi;
    TextView text_decaffeine;

    List<CoffeeOrder> listCart = new ArrayList<>();
    CartAdapter adapter;
    CartActivity cartActivity;

    public int cartQuantity=0;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_coffee_details);

        toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        database = FirebaseDatabase.getInstance();
        coffee = database.getReference("Coffee");

        coffee_name = findViewById(R.id.coffee_name);
        coffee_price = findViewById(R.id.coffee_price);
        coffee_image = findViewById(R.id.img_coffee);

        collapsingToolbarLayout = findViewById(R.id.collapsing);

        collapsingToolbarLayout.setExpandedTitleTextAppearance(R.style.ExpandedAppbar);
        collapsingToolbarLayout.setCollapsedTitleTextAppearance(R.style.CollapsedAppbar);

        btn_add_cart = findViewById(R.id.btn_add_cart);

        group_choose_varieties = findViewById(R.id.group_choose_varieties);
        group_choose_sugar = findViewById(R.id.group_choose_sugar);


        text_buondi = findViewById(R.id.text_buondi);
        text_decaffeine = findViewById(R.id.text_decaffeine);

        rb2 = findViewById(R.id.rb2);
        rb3 = findViewById(R.id.rb3);


        Bundle extras = getIntent().getExtras();
        if(extras != null){
            coffeeName = extras.getString("coffeeName");
            coffeePrice = extras.getString("coffeePrice");
            coffeeImage = extras.getString("coffeeImage");
        }


        coffee_name.setText(coffeeName);
        collapsingToolbarLayout.setTitle(coffeeName);


        coffee_price.setText(coffeePrice + " \u20ac");
        Picasso.with(getBaseContext()).load(coffeeImage)
                .into(coffee_image);


        listCart = new Database(this).getCarts();

        adapter = new CartAdapter(cartActivity,listCart, this);


        int quantity =0;
        if (adapter.getItemCount() != 0){
            quantity += adapter.getItemCount();
            cartQuantity = quantity;
        }


        btn_add_cart.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                int isSelected1 = group_choose_varieties.getCheckedRadioButtonId();
                int isSelected2 = group_choose_sugar.getCheckedRadioButtonId();

                if(isSelected1 == -1 && isSelected2 == -1){
                    Toast.makeText(CoffeeDetails.this, "Πρέπει να επιλέξετε ποικιλία και ζάχαρη για να συνεχίσετε...", Toast.LENGTH_SHORT).show();
                    return;
                }else if(isSelected1 != -1 && isSelected2 == -1){
                    Toast.makeText(CoffeeDetails.this, "Πρέπει να επιλέξετε ζάχαρη για να συνεχίσετε...", Toast.LENGTH_SHORT).show();
                }else if(isSelected1 == -1 && isSelected2 != -1){
                    Toast.makeText(CoffeeDetails.this, "Πρέπει να επιλέξετε ποικιλία για να συνεχίσετε...", Toast.LENGTH_SHORT).show();
                }else {
                    new Database(getBaseContext()).addToCart(new CoffeeOrder(
                            ID,
                            coffeeId,
                            coffeeName,
                            "1",
                            coffeePrice
                    ));


                    Toast.makeText(getApplication(), "Το προϊόν προστέθηκε στο καλάθι σας!", Toast.LENGTH_SHORT).show();
                }



            }
        });


    }


    // Add shopping cart icon in toolbar
    public boolean onCreateOptionsMenu(Menu menu){
        getMenuInflater().inflate(R.menu.menu, menu);

        MenuItem menuItem = menu.findItem(R.id.cartFragment);
        View actionView = menuItem.getActionView();

        TextView cartBadgeTextView = (TextView)actionView.findViewById(R.id.cart_badge_text_view);


        cartBadgeTextView.setText(String.valueOf(cartQuantity));

        actionView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                onOptionsItemSelected(menuItem);
            }
        });


        return true;
    }


    public boolean onOptionsItemSelected(MenuItem item){
        int id = item.getItemId();


        if(id == R.id.cartFragment){

            Intent intent = new Intent(CoffeeDetails.this, CartActivity.class);
            startActivity(intent);

            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onCartItemDelete(int position) {

    }
}

My code of Cart Activity is:

public class CartActivity extends AppCompatActivity implements CartAdapter.OnCartListener{

    Toolbar toolbar;

    RecyclerView recyclerView;
    RecyclerView.LayoutManager layoutManager;

    public TextView txtTotalPrice;
    public Button btnPlace;

    List<CoffeeOrder> listCart = new ArrayList<>();
    CartAdapter adapter;

    public LinearLayout empty_list;

    FirebaseDatabase database;
    DatabaseReference requests;

    private FirebaseUser user;
    private DatabaseReference reference;
    private String userID;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_cart);

        toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        database = FirebaseDatabase.getInstance();
        requests = database.getReference("Requests");

        user = FirebaseAuth.getInstance().getCurrentUser();
        reference = FirebaseDatabase.getInstance().getReference("Users");
        userID = user.getUid();

        recyclerView = findViewById(R.id.listCart);
        recyclerView.setHasFixedSize(true);
        layoutManager = new LinearLayoutManager(this);;
        recyclerView.setLayoutManager(layoutManager);

        txtTotalPrice = findViewById(R.id.total);
        btnPlace = findViewById(R.id.btnPlaceOrder);

        listCart = new Database(this).getCarts();

        adapter = new CartAdapter(this,listCart, this);
        recyclerView.setAdapter(adapter);

        empty_list = findViewById(R.id.empty_list);


        //Calculate total price
        Double double_total;
        double_total = 0.00;
        for(CoffeeOrder order:listCart) {
            double_total += (Double.parseDouble(order.getPrice()))*(Double.parseDouble(order.getQuantity()));

        }

        String total = String.format("%.2f", double_total);

        txtTotalPrice.setText(total + " \u20ac");

        // check if cart list is empty. If cart list is empty, then disable button
        if (adapter.getItemCount() == 0){

            //making it semi-transparent
            btnPlace.setAlpha(.5f);
            btnPlace.setEnabled(false);
            empty_list.setVisibility(LinearLayout.VISIBLE);
        }


        btnPlace.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                showAlertDialog();
            }
        });



    }

    private void showAlertDialog(){
        AlertDialog.Builder builder = new AlertDialog.Builder(CartActivity.this);

        EditText edtAddress = new EditText(this);
        LinearLayout.LayoutParams editTextParams = new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.MATCH_PARENT,
                LinearLayout.LayoutParams.MATCH_PARENT);

        TextInputLayout textInputLayout = new TextInputLayout(this, null, R.style.Widget_MaterialComponents_TextInputLayout_OutlinedBox);

        LinearLayout.LayoutParams textInputLayoutParams = new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.MATCH_PARENT,
                LinearLayout.LayoutParams.WRAP_CONTENT);

        textInputLayout.setLayoutParams(textInputLayoutParams);
        textInputLayout.addView(edtAddress, editTextParams);
        textInputLayout.setHint("Address");
        edtAddress.setTextSize(15);
        editTextParams.setMargins(60,30,60,80);

        builder.setCancelable(false);
        builder.setView(textInputLayout);
        builder.setTitle("Ένα βήμα ακόμα");
        builder.setMessage("Συμπλήρωσε τη διεύθυνση σου για την ολοκλήρωση της παραγγελίας σου!");
        builder.setPositiveButton("Συνέχεια", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                if (TextUtils.isEmpty(edtAddress.getText().toString())) {
                    Toast.makeText(CartActivity.this, "Το πεδίο είναι απαραίτητο", Toast.LENGTH_SHORT).show();
                }else{
                    user = FirebaseAuth.getInstance().getCurrentUser();
                    reference = FirebaseDatabase.getInstance().getReference("Users");
                    userID = user.getUid();

                    Request request = new Request(
                            userID,
                            edtAddress.getText().toString(),
                            txtTotalPrice.getText().toString(),
                            listCart
                    );

                    //Submit to Firebase
                    requests.child(String.valueOf(System.currentTimeMillis()))
                            .setValue(request);

                    //Delete Cart
                    new Database(getBaseContext()).cleanCart();
                    Toast.makeText(getApplication(),"Σας ευχαριστούμε για την παραγγελία σας!", Toast.LENGTH_SHORT).show();
                    startActivity(new Intent(CartActivity.this, MainFragment.class));
                    finish();
                }

            }
        });

        builder.setNegativeButton("Ακύρωση", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                dialog.dismiss();
            }
        });

        AlertDialog alertDialog = builder.create();
        alertDialog.show();


    }


    @Override
    public void onCartItemDelete(int position) {
    }
}

My xml code of cart axctivity is:

<androidx.coordinatorlayout.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".CartActivity">

    <com.google.android.material.appbar.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">

        <androidx.appcompat.widget.Toolbar
            app:title="Το καλάθι σας"
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="@color/black"
            app:layout_scrollFlags="scroll|enterAlways"
            app:popupTheme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"/>

    </com.google.android.material.appbar.AppBarLayout>


    <androidx.core.widget.NestedScrollView
        android:id="@+id/nestedScrollView"
        android:clipToPadding="false"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@drawable/cart_background"
        app:layout_behavior="@string/appbar_scrolling_view_behavior">

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            >

            <androidx.recyclerview.widget.RecyclerView
                android:id="@+id/listCart"
                android:background="@android:color/transparent"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:layout_marginBottom="105dp"/>

        </LinearLayout>
    </androidx.core.widget.NestedScrollView>

    <LinearLayout
        android:id="@+id/empty_list"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:orientation="vertical"
        android:visibility="gone"
        tools:visibility="invisible">

        <ImageView
            android:id="@+id/empty_image"
            android:layout_width="100dp"
            android:layout_height="100dp"
            android:src="@drawable/ic_empty_box" />

        <TextView
            android:id="@+id/emptyMessage"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Η λίστα σας είναι κενή!"
            android:textSize="15sp"
            android:alpha="0.7"
            android:textColor="@color/black" />


    </LinearLayout>

    <androidx.cardview.widget.CardView
        android:layout_gravity="bottom"
        android:layout_marginStart="5dp"
        android:layout_marginEnd="5dp"
        android:layout_marginBottom="5dp"
        android:layout_alignParentBottom="true"
        app:cardBackgroundColor="@color/register_background"
        android:layout_width="match_parent"
        android:layout_height="100dp">

        <RelativeLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent">

            <LinearLayout
                android:layout_margin="8dp"
                android:gravity="center"
                android:orientation="horizontal"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content">

                <TextView
                    android:text="Συνολικό ποσό: "
                    android:textSize="20sp"
                    android:textColor="@color/black"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />

                <TextView
                    android:id="@+id/total"
                    android:textSize="18sp"
                    android:textColor="@color/black"
                    android:textStyle="bold"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content" />

            </LinearLayout>

            <Button
                android:id="@+id/btnPlaceOrder"
                android:text="Ολοκλήρωση Παραγγελίας"
                android:textColor="@color/black"
                android:layout_marginRight="8dp"
                android:backgroundTint="@color/yellow"
                android:layout_marginLeft="8dp"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="40dp"
                android:textAllCaps="false"
                />


        </RelativeLayout>

    </androidx.cardview.widget.CardView>



</androidx.coordinatorlayout.widget.CoordinatorLayout>