Flutter mock http.Client always return NUll

Question

On Flutter unit testing i am following the same tutorial from https://docs.flutter.dev/cookbook/testing/unit/mocking , but when try to run test . The http.Client always return null. how to solve this? Generated Mock Client

 @GenerateMocks([
  http.Client
], customMocks: [
  MockSpec<NavigationService>(returnNullOnMissingStub: true),
  MockSpec<UserService>(returnNullOnMissingStub: true),
  MockSpec<DialogService>(returnNullOnMissingStub: true),
  MockSpec<AlbumService>(returnNullOnMissingStub: true)
])

 test('returns an Album if the http call completes successfully', () async {
      final client = MockClient();
      final albumService = getAndRegisterAlbumService();
      when(client
              .get(Uri.parse('https://jsonplaceholder.typicode.com/albums/1')))
          .thenAnswer((_) async =>
              http.Response('{"userId": 1, "id": 2, "title": "mock"}', 200));

      // expect(await albumService.fetchAlbum(client), isA<Album>());

      expect(await albumService.fetchAlbum(client), isA<Album>());
    });