Stubbed function always returns null - flutter

Question

Currently I'm following a tutorial of Resocoder (DDD Course). In this course there is an abstract class called i_auth_facade.dart, which contains a function getSignedInUser.
I have mocked the class and trying to get a valid option of user back from the getSignedInUser method, but the result is always null.
Also when I try to execute the authCheckRequested method of the AuthBloc, which then in the background invokes the getSignedInUser method, I get back null.
I must have some error with Mockito, as it seems the stub doesn't work.
Can anybody point me into the right direction why I don't get back the optionOf(User), but null instead?
import 'package:bloc_test/bloc_test.dart';
import 'package:dartz/dartz.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/mockito.dart';
import 'package:teach_mob_student/application/auth/auth_bloc.dart';
import 'package:teach_mob_student/domain/auth/i_auth_facade.dart';
import 'package:teach_mob_student/domain/auth/user.dart';
import 'package:teach_mob_student/domain/auth/value_objects.dart';
import 'package:teach_mob_student/domain/core/value_objects.dart';
class MockIAuthFacade extends Mock implements IAuthFacade {}
void main() {
final tValidUser = User(
id: UniqueId(),
name: StringSingleLine('Test User'),
emailAddress: EmailAddress('test#user.com'));
test('Should return valid option of user', () async {
// This is to check, if optionOf actually returns a user --> Yes it does
Option<User> tDirectResult = optionOf<User>(tValidUser);
// Here I stub the getSignedInUser method. If it is executed, I want to receive a valid user.
when(MockIAuthFacade()
.getSignedInUser()
).thenAnswer((_) async => Future.value(optionOf<User>(tValidUser)));
// Here I execute the method, and I would expect to get Future<Option<User>> back, but I only get back null.
final facade = MockIAuthFacade();
Option<User> tStubbedResult = await facade.getSignedInUser();
print(tDirectResult);
print(tStubbedResult);
});
// Here I do the same, but I execute the authCheckRequested method of the AuthBloc, which executes the
// getSignedInUser method. Also here I get back null as result.
blocTest('Should return valid option of user.',
build: () async {
when(MockIAuthFacade()
.getSignedInUser()
).thenAnswer((_) async => Future.value(optionOf<User>(tValidUser)));
return AuthBloc(MockIAuthFacade());
},
act: (bloc) async {
bloc.add(AuthEvent.authCheckRequested());
},
expect: [
]
);
}