Android - NullPointerException on setting Adapter

Question

Im trying to set up a project with an navigation drawer as a central feature. It is supposed to show the names of all files in the apps internal file directory. To update that list i figured it would be best overriding the onDrawerOpen method of the ActionBarToggle. The code of the main activity:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close){
        @Override
        public void onDrawerOpened(View view){
            super.onDrawerOpened(view);
            File f = new File(getFilesDir().toString());
            File[] files = f.listFiles();
            String[] testString = {"Text1","Text2","Text3"};
            String[] fileNames = new String[files.length];
            for (int i = 0; i < files.length; i++) {
                fileNames[i] = files[i].getName();
            }
            ListView listView = (ListView)findViewById(R.id.file_list);
            listView.setAdapter(new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_list_item_1, testString));
        }
        //Toast.makeText(MainActivity.this,fileNames[0], Toast.LENGTH_SHORT).show();
        @Override
        public void onDrawerClosed(View view){
            super.onDrawerClosed(view);

        }

    };
    drawer.setDrawerListener(toggle);
    toggle.syncState();

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);
}

And the xml files:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">
    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        app:popupTheme="@style/AppTheme.PopupOverlay" />
</android.support.design.widget.AppBarLayout>
<android.support.v4.widget.DrawerLayout 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:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:openDrawer="start">

    <include
        layout="@layout/app_bar_main"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <android.support.design.widget.NavigationView
        android:id="@+id/nav_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:fitsSystemWindows="true"
        app:headerLayout="@layout/nav_header_main"
        app:menu="@menu/activity_main_drawer" />

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

The referenced activity_main_drawer menu looks like this

<group android:checkableBehavior="single">
    <item
        android:id="@+id/nav_camera"
        android:icon="@drawable/ic_menu_camera"
        android:title="Import" />
    <item
        android:id="@+id/nav_gallery"
        android:icon="@drawable/ic_menu_gallery"
        android:title="Gallery" />
    <item
        android:id="@+id/nav_slideshow"
        android:icon="@drawable/ic_menu_slideshow"
        android:title="Slideshow" />
    <item
        android:id="@+id/nav_manage"
        android:icon="@drawable/ic_menu_manage"
        android:title="Tools" />
</group>

<item android:title="Communicate">
    <menu>
        <item
            android:id="@+id/nav_share"
            android:icon="@drawable/ic_menu_share"
            android:title="Share" />
        <item
            android:id="@+id/nav_send"
            android:icon="@drawable/ic_menu_send"
            android:title="Send" />
    </menu>
</item>


<group android:id="@+id/gruppe">
    <ListView android:id="@+id/file_list"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:choiceMode="singleChoice"
        android:divider="@android:color/transparent"
        android:dividerHeight="0dp"
        android:background="#111"/>
</group>

So im trying to set the adapter to the ListView with id "file_list" but i recieve an nullPointerException to that ListView. Can anybody please tell me what im doing wrong? Thanks in advance!