Flutter: How to use GetIt.I in Integration test - flutter

Question

I am trying to use GetIt in my integration test but I am having some difficulties.
When trying to access registered singletons in GetIt, I am always facing this error:
_AssertionError ('package:get_it/get_it_impl.dart': Failed assertion: line 370 pos 7: 'instanceFactory != null': Object/factory with type DrawScreenState is not registered inside GetIt.
(Did you accidentally do GetIt sl=GetIt.instance(); instead of GetIt sl=GetIt.instance;
Did you forget to register it?))
I have a function to register my singletons:
// Initialize GetIt by initializing and registering all the instances for it
Future<void> initGetIt() async {
...
// draw screen services
GetIt.I.registerSingleton<DrawScreenState>(DrawScreenState(
Strokes(), KanjiBuffer(), DrawingLookup(), DrawScreenLayout.Portrait)
);
...
}
This function is called when creating my app instance. However, when I call my function, the singletons are not registered in GetIt and the above Exception is thrown. My code looks like this:
void main() {
IntegrationTestWidgetsFlutterBinding.ensureInitialized();
testWidgets("DrawScreen test", (WidgetTester tester) async {
IS_TESTING_DRAWSCREEN = true;
// create app instance and wait until it finished initializing
app.main(); // <-- this calls initGetIt()
print(GetIt.I<DrawScreenState>()); // <-- this raises the exception
await tester.pumpAndSettle(Duration(seconds: 2));
// check that the app does not show any predictions on start up
List<String> preds = (tester.widgetList(find.byType(PredictionButton))).map((e) => (e as PredictionButton).char).toList();
...
}
}
In the README integration tests are mentioned, therefore I assume that it should be possible to access GetIt in them. This leads me to the question what am I doing wrong?