Unhandled Exception: inheritFromWidgetOfExactType(_LocalizationsScope) or inheritFromElement() was called before _ScreenState.initState() completed

You need to call _getCategories after initState has completed.

@override
void initState() {
   super.initState();

   Future.delayed(Duration.zero, () {
      this._getCategories();
   });

   // Could do this in one line: Future.delayed(Duration.zero, this._getCategories);
}

Also, you could do this on a different way, using addPostFrameCallback like showed in another answers.

Use the didChangeDependencies method which gets called after initState.

For your example:

@override
void initState() {
  super.initState();
}

@override
void didChangeDependencies() {
  super.didChangeDependencies();

  this._getCategories();
}

void _getCategories() async {
  // Omitted for brevity
  // ...

 }

Adding a frame callback might be better than using Future.delayed with a zero duration - it's more explicit and clear as to what is happening, and this kind of situation is what frame callback was designed for:

@override
  void initState() {
    super.initState();

    WidgetsBinding.instance.addPostFrameCallback((_) async {
      _getCategories();
    });
  }