Flutter widget testing: Button tap won't work - flutter

Question

I'm using widget testing in Flutter. I'm using the game template from here
https://github.com/flutter/samples/tree/main/game_template
and my test doesn't work as expected:
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:escape_team/main.dart';
void main() {
testWidgets('Main menu can be started', (WidgetTester tester) async {
// Build our app and trigger a frame.
await tester.pumpWidget(MyApp(
inAppPurchaseController: null,
playerProgressPersistence: null,
settingsPersistence: null,
));
expect(find.text('PLAY'), findsOneWidget);
await tester.tap(find.text('PLAY'));
await tester.pumpAndSettle();
expect(find.text('MISSION SELECT'), findsOneWidget);
});
}
results in:
00:00 +0: Main menu can be started
══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following TestFailure was thrown running a test:
Expected: exactly one matching node in the widget tree
Actual: _TextFinder:<zero widgets with text "MISSION SELECT" (ignoring offstage widgets)>
Which: means none were found but one was expected
However, when I manually tap the 'PLAY' button in my app, the "MISSION SELECT" screen appears. How can I find out what's going on - and wrong?
UPDATE:
I did some more research, I guess it's caused by a JSON that's being loaded by my app before the main menu is started.
missionLoader
.readJson('assets/json/missions_en.json')
.then((value) => GoRouter.of(context).go('/play'));
This calls
final String response = await rootBundle.loadString(jsonFile);
... and rootBundle file loading isn't supported out of the box in Flutter tests, as I see it?
I tried adding
WidgetsFlutterBinding.ensureInitialized();
to my app's main() method, as well as to the test's main method, but both did not help. Any further leads would be greatly appreciated! Thank you!