setSupportActionBar(toolbar); is not wworking

Question

My app continuously says "Unfortunately stopped". So I went through single line and found that the error is in below line

setSupportActionBar(toolbar);

I have searched here for some related post about this and they say to add this line

import androidx.appcompat.widget.Toolbar;

But I have added it also. Here is my code

package com.locationtracker2019;

import android.os.Bundle;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.firebase.ui.database.FirebaseRecyclerAdapter;
import com.google.firebase.database.DatabaseReference;

public class ListOnline extends AppCompatActivity {

    //Firebase
    DatabaseReference onlineRef, currentUserRef, counterRef;
    FirebaseRecyclerAdapter<User, ListOnlineViewHolder> adapter;

    //View
    RecyclerView listOnline;
    RecyclerView.LayoutManager layoutManager;

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

        //Init View
        listOnline = (RecyclerView)findViewById(R.id.listOnline);
        listOnline.setHasFixedSize(true);
        layoutManager = new LinearLayoutManager (this);
        listOnline.setLayoutManager(layoutManager);

        //set Toolbar and Logout / Join menu
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolBar);
        toolbar.setTitle("LocationTracker2019");                    //up to here everything is fine
        setSupportActionBar(toolbar);


        //Firebase
        onlineRef = FirebaseDatabase.getInstance().getReference().child(".info/connected");
        counterRef = FirebaseDatabase.getInstance().getReference("lastOnline");     //Create new child name lastOnline
        currentUserRef = FirebaseDatabase.getInstance().getReference("lastOnline")
                .child(FirebaseAuth.getInstance().getCurrentUser().getUid());       //Create new child in lastOnline with key is uid

        setUpSystem();
        //after setup system,  just load all user from counterRef and display on RecyclerView
        //this is online list
        updateList();
    }

    private void updateList() {
        adapter = new FirebaseRecyclerAdapter<User, ListOnlineViewHolder>(
                User.class,
                R.layout.user_layout,
                ListOnlineViewHolder.class,
                counterRef
        ) {
            @Override
            protected void populateViewHolder(ListOnlineViewHolder listOnlineViewHolder, User user, int i) {
                listOnlineViewHolder.txtEmail.setText(user.getEmail());
            }
        };
        adapter.notifyDataSetChanged();
        listOnline.setAdapter(adapter);
    }

    private void setUpSystem() {
        onlineRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                if(dataSnapshot.getValue(Boolean.class)){
                    currentUserRef.onDisconnect().removeValue();
                    //set online user in list
                    counterRef.child(FirebaseAuth.getInstance().getCurrentUser().getUid())
                            .setValue(new User(FirebaseAuth.getInstance().getCurrentUser().getEmail(), "Online"));
                    adapter.notifyDataSetChanged();
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
        counterRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                for(DataSnapshot postSnapshot:dataSnapshot.getChildren())
                {
                    User user = postSnapshot.getValue(User.class);
                    Log.d("LOG",""+user.getEmail()+" is "+user.getStatus());
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.main_menu, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
        switch (item.getItemId())
        {
            case R.id.action_join:
                counterRef.child(FirebaseAuth.getInstance().getCurrentUser().getUid())
                        .setValue(new User(FirebaseAuth.getInstance().getCurrentUser().getEmail(), "Online"));
            break;
            case R.id.action_logout:
                currentUserRef.removeValue();
            break;
        }
        return super.onOptionsItemSelected(item);
    }
}

Answer 1

It's hard to say the actual problem without stack trace but it seems like

IllegalStateException: This Activity already has an action bar

If so then you have to use NoActionBar variant of theme for the Activity / Application

Theme.AppCompat.Light.NoActionBar

Check official documentation about that.