Fragment in Navigation Drawer not loading and makes the app crash. What can I do?

Question

So, I built a Navigation Drawer for my program with the following code:

This is the main xml:

<android.support.v4.widget.DrawerLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".HomeScreen">

        <include
            android:id="@+id/eb_bar"
            layout="@layout/eb_bar" />

        <FrameLayout
            android:id="@+id/main_screen"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_below="@+id/eb_bar"
            android:background="@color/frame_layout_bg"/>

    </RelativeLayout>

    <fragment
        android:id="@+id/frag_navigation_drawer"
        android:layout_width="@dimen/navigation_drawer_width"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        app:layout="@layout/fragment_navigation_drawer"
        android:name="com.myapps.project.NavigationDrawerFragment"
        tools:layout="@layout/fragment_navigation_drawer"
        />

</android.support.v4.widget.DrawerLayout>

This is the xml for the Navigation Drawer:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/navigation_drawer_background"
    tools:context="com.myapps.project.NavigationDrawerFragment">

    <FrameLayout
        android:id="@+id/nav_drawer_frame_logo"
        android:layout_width="match_parent"
        android:layout_height="160dp"
        android:background="@color/splash_screen_bg"
        android:elevation="3dp">

        <ImageView
            android:id="@+id/nav_drawer_logo"
            android:layout_width="90dp"
            android:layout_height="wrap_content"
            android:layout_marginLeft="@dimen/navigation_drawer_center_logo"
            android:layout_marginStart="@dimen/navigation_drawer_center_logo"
            android:src="@drawable/logo"
            tools:ignore="ContentDescription" />
    </FrameLayout>

    <ListView
        android:id="@+id/nav_drawer_menu_ListView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/nav_drawer_frame_logo"
        android:choiceMode="singleChoice"
        android:divider="@color/divider_color"
        android:dividerHeight="1dp"
        />

</RelativeLayout>

And this is xml of the fragment that I want to load:

<FrameLayout
 xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/Second_screen"
    android:tag="Second"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#00F"
    tools:context="com.myapps.project.Second_Screen"
    android:name="com.myapps.project.Second_Screen"
    tools:layout="@layout/fragment_second_screen">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_second_screen" />

</FrameLayout>

I also have two .java. One for the main screen and one for the Navigation Drawer. On the main screen I have this code:

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home_screen);


        toolBar = (Toolbar) findViewById(R.id.eb_bar);
        setSupportActionBar(toolBar);

        getSupportActionBar().setTitle(null);

        TextView toolbar_home_title = (TextView) findViewById(R.id.home_toolbar_textview);
        toolbar_home_title.setText(R.string.home_screen_title);
        toolbar_home_title.setTextColor(getResources().getColor(R.color.toolbar_txt_color));

        toolBar.setBackgroundColor(getResources().getColor(R.color.toolbar_bg));

        NavigationDrawerFragment drawerFragment = (NavigationDrawerFragment)
                getSupportFragmentManager().findFragmentById(R.id.frag_navigation_drawer);

        navDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawerFragment.setUp(R.id.frag_navigation_drawer,
                navDrawerLayout, toolBar);

        navDrawerMenuItems = getResources().getStringArray(R.array.navigation_drawer_items);

        navDrawerListView = (ListView) findViewById(R.id.nav_drawer_menu_ListView);

        FragNavigationDrawer = (FrameLayout) findViewById(R.id.frag_navigation_drawer);

        navDrawerListView.setAdapter(new ArrayAdapter<String>(
                this,
                R.layout.navigation_drawer_list_item,
                navDrawerMenuItems));

        navDrawerListView.setOnItemClickListener(new DrawerItemClickListener());
    }

    public class DrawerItemClickListener implements ListView.OnItemClickListener {
        @Override
        public void onItemClick(AdapterView parent, View view, int position, long id) {
            selectItem(position);
        }
    }

    public void selectItem(int position) {

        String a = navDrawerMenuItems[position];

        Fragment navFragment = getSupportFragmentManager().findFragmentByTag(a);

        android.support.v4.app.FragmentManager fragmentManager = getSupportFragmentManager();
        fragmentManager.beginTransaction()
                .replace(R.id.main_screen, navFragment)
                .commit();

        navDrawerListView.setItemChecked(position,true);
        navDrawerLayout.closeDrawer(FragNavigationDrawer);
    }

The navDrawerMenuItems array has the names that are loaded into the Drawer to be the clickable items. They're also the tags of each fragment I want to load on the screen. So, what I'm trying to do inside the selectItem is to load the fragment by its tag, replace it on the main screen and close the drawer at the end. But this code is showing the error:

2449-2449/com.myapps.project E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.myapps.project, PID: 2449
java.lang.NullPointerException: Attempt to write to field 'android.support.v4.app.FragmentManagerImpl android.support.v4.app.Fragment.mFragmentManager' on a null object reference
        at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:416)
        at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:451)
        at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:443)
        at com.myapps.project.MainScreen.selectItem(MainScreen.java:106)
        at com.myapps.project.MainScreen$DrawerItemClickListener.onItemClick(MainScreen.java:88)
        at android.widget.AdapterView.performItemClick(AdapterView.java:300)
        at android.widget.AbsListView.performItemClick(AbsListView.java:1143)
        at android.widget.AbsListView$PerformClick.run(AbsListView.java:3044)
        at android.widget.AbsListView$3.run(AbsListView.java:3833)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

What am I doing wrong here?

Answer 1

You should import android.support.v4.app.FragmentManager not android.app.FragmentManager. Then try this

FragmentManager fragmentManager = getSupportFragmentManager();

Update

Can You check this?

<fragment
    android:id="@+id/frag_navigation_drawer">

The above line in xml is id for fragment. And you are casting it as frame layout in java file.

FragNavigationDrawer = (FrameLayout) findViewById(R.id.frag_navigation_drawer);