Why is my fragment still null even though I added by the fragment manager?

MainActivivty.java


    public class MainActivity extends AppCompatActivity implements FragmentA.Communicator {


    FragmentManager manager;
    FragmentA f1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        manager = getSupportFragmentManager();

        Fragment fragment = manager.findFragmentById(R.id.main_fragment_container);
        if (fragment == null) {
            fragment = new FragmentA();
            manager.beginTransaction()
                    .add(R.id.main_fragment_container, fragment)
                    .commit();
        }

        f1 = (FragmentA) manager.findFragmentById(R.id.main_fragment_container);
        f1.setCommunicator(this);

    }

    @Override
    public void changeText(String text) {
        FragmentB f2 = (FragmentB) manager.findFragmentById(R.id.details_fragment_container);

        if (f2 != null && f2.isVisible()) {
            f2.updateText(text);
        } else {
            Intent intent = new Intent(MainActivity.this, DetailsActivity.class);
            intent.putExtra("text", text);
            startActivity(intent);
        }
    }
}


<big>activity_main.xml</big>

    <?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <FrameLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/main_fragment_container" />

</LinearLayout>


DetailsActivity.java 

    public class DetailsActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_details);

        Intent intent = getIntent();
        String recievedText = intent.getStringExtra("text");
        FragmentB f2 = (FragmentB) getSupportFragmentManager().findFragmentById(R.id.details_fragment_container);
        if (f2 != null) {
            f2.updateText(recievedText);
        }

    }
}



details_activity.xml

    <?xml version="1.0" encoding="utf-8"?>
    <LinearLayout xmlns:android="http://schemas.dandroid.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".DetailsActivity">

    <fragment
        android:id="@+id/details_fragment_container"
        android:layout_width="wrap_content"
        android:name="com.example.workout.FragmentB"
        android:layout_height="wrap_content" />

</LinearLayout>



    FragmentA.java

    public class FragmentA extends Fragment {

    Communicator communicator;
    ListView listView;

    public FragmentA() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment


        View view = inflater.inflate(R.layout.fragment_a, container, false);
        listView = view.findViewById(R.id.listview);
        return view;
    }

    public void setCommunicator(Communicator communicator) {
        this.communicator = communicator;
    }

     @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        final ArrayList<String> arrayList = new ArrayList<>();
        arrayList.add("first row");
        arrayList.add("second row");
        arrayList.add("third row");

        ArrayAdapter<String> adapter = new ArrayAdapter<>(getActivity(), android.R.layout.simple_list_item_1,
                arrayList);
        listView.setAdapter(adapter);

        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                communicator.changeText(arrayList.get(position));
            }
        });
    }

    public interface Communicator {
        void changeText(String text);
    }
}


fragment_a.xml

    <?xml version="1.0" encoding="utf-8"?>
<FrameLayout 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"
    tools:context=".FragmentA">

 <ListView
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:id="@+id/listview" />

</FrameLayout>

Log:

    
       java.lang.NullPointerException: Attempt to invoke virtual method 'void 
        com.example.workout.FragmentA.setCommunicator(com.example.workout.FragmentA$Communicator)' 
        on a null object reference
         Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void 
        com.example.workout.FragmentA.setCommunicator(com.example.workout.FragmentA$Communicator)' 
        on a null object reference
        at com.example.workout.MainActivity.onCreate(MainActivity.java:33)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)