Can't change the TextView in the first fragment of Basic Activity template

Question

I just created a new Basic Activity from the gallery of Android Studio and I can't figure out how to change the text of the fragment_first. I've tried this, this, this and this but nothing works, I can't change the text of the fragment from the MainActivity.java file. I always end up getting a "null reference" error, and it doesn't make sense to me.

Here are my files, they're basically the same as the Basic Activity template, but with a new method in the FirstFragment.java that should change the text inside it.

------MainActivity.java------

public class MainActivity extends AppCompatActivity {
    FirstFragment firstFragment;

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

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        //Here's my problem. 
        firstFragment = (FirstFragment) getSupportFragmentManager().findFragmentById(R.id.firstfrag);
        firstFragment.changeTextOfFragment("Stuff");
    }

    //onCreateOptionsMenu, onOptionsItemSelected...
}

------FirstFragment.java------

public class FirstFragment extends Fragment {
    private View view;
    private TextView textView;

    @Override
    public View onCreateView(
            LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState
    ) {
        // Inflate the layout for this fragment
        view = inflater.inflate(R.layout.fragment_first, container, false);
        //On debug mode this works, it assigns the view to the textView variable
        textView = view.findViewById(R.id.textview_first);
        return view;
    }

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        view.findViewById(R.id.button_first).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                NavHostFragment.findNavController(FirstFragment.this)
                        .navigate(R.id.action_FirstFragment_to_SecondFragment);
            }
        });
    }

    // My method
    public void changeTextOfFragment(String text) {
         textView = view.findViewById(R.id.textview_first);
         textView.setText(text);
    }
}

And finally, here's the XML for the FirstFragment:

------FirstFragment.xml------

<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:id="@+id/firstfrag"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".FirstFragment">

    <TextView
        android:id="@+id/textview_first"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_first_fragment"
        app:layout_constraintBottom_toTopOf="@id/button_first"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
           .../>
</androidx.constraintlayout.widget.ConstraintLayout>

I think that the only possible logical reason as to why I'm referencing a null fragment it's because the fragment itself is finishing being created only after the MainActivity is already done. If so, how can I be sure the fragment was finally created before trying to change its text? Should I set the TextView of the FirstFragment class on the onCreateView() method or onViewCreated(), since the button is being referenced there?