Flutter Unit test: Bad state: No method stub was called from within `when()`. Was a real method called, or perhaps an extension method?

I am trying to mock a stream in my unit test (using mocktail) that comes from a third-party library:

This my test code:

void main() {
  late AirPodsMotionProvider airPodsMotionProvider;
  final mockDeviceMotionData = DeviceMotionDataMocks();

  setUp(() {
    WidgetsFlutterBinding.ensureInitialized();
    airPodsMotionProvider = AirPodsMotionProvider();
  });

  group('test AirPodsMotionProvider', () {
    group('test listenForAirPodsMotionData', () {
      test('test listen for incoming data', () async {
        final value = mockDeviceMotionData.mockDefaultDeviceMotionData;

        when(
          () => airPodsMotionProvider.listenForAirPodsMotionData(),
        ).thenAnswer((_) => Stream<DeviceMotionData>.value(value));

        airPodsMotionProvider.listenForAirPodsMotionData().listen(
          expectAsync1(
            (event) {
              expect(event, value);
            },
          ),
        );
      });
    });
  });
}

I have written a provider which wraps the third party's library functionality called AirPodsMotionProvider and it looks like this:

class AirPodsMotionProvider {
  Stream<DeviceMotionData> listenForAirPodsMotionData() =>
      FlutterAirpods.getAirPodsDeviceMotionUpdates;
}

My question is how can I make the stream return the stream with a specific value.
I read some answers mentioning I need to do something like:

class MockFlutterAirpods extends Mock implements FlutterAirPods {}

And then make that function return a specific function, but it didn't work for me exactly.

Your airPodsMotionProvider must be a mock, this Mock child class so that you will be able to stub its methods with when.

So you need a mocked class:

class MockAirPodsMotionProvider extends Mock implements AirPodsMotionProvider {}

and then in your setUp you initialize your airPodsMotionProvider with such class - MockAirPodsMotionProvider.

The further call to when will then work as expected. But what you do in this test is you actually test the mock. You may want to pass this mock to a class that depends on the AirPodsMotionProvider to test this class's behavior, not the mock itself, as it's just a mock. It doesn't have the logic of what it mocks.