Android toolbar menu items remain empty

My Android App has a Toolbar to which I want to add menu items. The Toolbar is displayed correctly, but when expanding the menu the menu items are void.

The menu structure is defined in toolbar_menu.xml:

toolbar_menu.xml

<menu 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/about"
        android:title="About"
        app:showAsAction="never"/>
</menu>

The activity defines the toolbar in the layout file activity_layout.xml

activity_layout.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
    android:visibility="visible"
    tools:context=".MainActivity">

    <androidx.appcompat.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="?attr/colorPrimary"
        android:minHeight="?attr/actionBarSize"
        android:theme="?attr/actionBarTheme"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:title="Welcome"/>

    <!-- other widgets follow -->

</androidx.constraintlayout.widget.ConstraintLayout>

And the activity class where I set the Toolbar is MainActivity.java

MainActivity.java

// import dependencies

public class MainActivity extends AppCompatActivity {

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

        final Toolbar toolbar = (Toolbar) this.findViewById(R.id.toolbar);
        this.setSupportActionBar(toolbar);
    }




    @Override
    public boolean onCreateOptionsMenu(final Menu menu) {
        this.getMenuInflater().inflate(R.menu.toolbar_menu, menu);
        return true;
    }
}