My App is giving me a NullPointerException. Why?

Question

I was recently working on a todo list app.

The code for the app's OnCreate method is shown below:

public class MainActivity extends AppCompatActivity {

    ArrayList<String> items;
    ArrayAdapter<String> itemsAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                WindowManager.LayoutParams.FLAG_FULLSCREEN);
        setContentView(R.layout.activity_main);

        ListView todo = (ListView) findViewById(R.id.list);
        items = new ArrayList<>();
        itemsAdapter = new ArrayAdapter<>(this,android.R.layout.simple_list_item_1, items);
        todo.setAdapter(itemsAdapter);

        ActionBar ab = getSupportActionBar();
        assert ab != null;
        ab.hide();



    }

But when I run the app, it keeps crashing.

I checked the logcat to see what the error was and I got the following piece.

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dobleu.hotlists/com.dobleu.hotlists.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference

I have no idea why I'm getting a NPE.

I am using fragments and I am utilizing a BottomNavigationView to transition between them.

Here is the xml for the fragment with the listview:

<android.support.constraint.ConstraintLayout
    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">

    <LinearLayout
        android:layout_width="0dp"
        android:layout_height="232dp"
        android:background="@drawable/header1"
        android:scaleType="fitStart"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:id="@+id/imageView"
        android:orientation="vertical"/>

    <LinearLayout
        android:layout_width="0dp"
        android:layout_height="50dp"
        app:layout_constraintTop_toBottomOf="@+id/imageView"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        android:background="#e95555"
        android:id="@+id/linearLayout"
        android:orientation="vertical">

    </LinearLayout>

    <ListView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/list"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        tools:layout_editor_absoluteX="0dp"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/linearLayout" />

    <android.support.design.widget.FloatingActionButton
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginRight="13dp"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        android:layout_marginBottom="8dp"
        android:background="#e95555"
        app:fabSize="normal"
        android:onClick="AddItem"/>

</android.support.constraint.ConstraintLayout>

And here's the xml for the main activity:

    <RelativeLayout 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="com.dobleu.hotlists.MainActivity">

    <FrameLayout
        android:id="@+id/frame_layout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_above="@+id/navigation"
        android:animateLayoutChanges="true">


    </FrameLayout>


    <android.support.design.widget.BottomNavigationView
        android:id="@+id/navigation"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:background="#2b2b2b"
        app:itemIconTint="#fff"
        app:itemTextColor="#fff"
        app:menu="@menu/menu"/>
</RelativeLayout>

I have tried a lot to get it working but I have not been able to find the solution.

I've read that post on what a NPE is and why, but I still don't understand.

And now I'm sick and tired that people keep marking my questions as duplicates when I am very well aware that there is an answer that can apply to this question but I have no idea how to apply it.

Please help me.