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.
To make this task easier, you could create a mixin to be added to StatefulWidgets.

mixin PostFrameMixin<T extends StatefulWidget> on State<T> {
  void postFrame(void Function() callback) =>
      WidgetsBinding.instance?.addPostFrameCallback(
        (_) {
          // Execute callback if page is mounted
          if (mounted) callback();
        },
      );
}

Then, you just need to plug this mixin to you page, like that:

class _MyPageState extends State<MyPage> with PostFrameMixin {
  @override
  void initState() {
    super.initState();

    postFrame(_getCategories);
  }
}