How to return state from mocked provider on widget testing

[tazik561]

this is my provider

@riverpod
class MainProductCategoriesNew extends _$MainProductCategoriesNew {
  Future<void> getMainProductCategories() async {
    state = const UIState.loading();
    final result = await ref.watch(...);...
    result.when(
      data: (data) {
        state = UIState<List<MainProductCategoryModel>>.success(data: data);
      },
      error: (e, s) {
 
      },
    );
  }

  @override
  UIState<List<MainProductCategoryModel>> build() {
    return const UIState.initial();
  }
}

I want to write a widget test so I have mocked MainProductCategoriesNew class:

@GenerateNiceMocks([
  MockSpec<MainProductCategoriesNew>(),

])
void main() {
  late MockMainProductCategoriesNew mainProductCategoriesNew;
  setUp(() {

    mainProductCategoriesNew = MockMainProductCategoriesNew();
  });

    testWidgets('Test',
        (tester) async {
      // Mock the behavior of getMainProductCategories
      when(mainProductCategoriesNew.getMainProductCategories())
          .thenAnswer((_) async {});

      // Mock the initial state
      when(mainProductCategoriesNew.state).thenReturn(const UIState.initial());

      await mockNetworkImages(
          () async => tester.pumpApp(const ExploreScreen(), overrides: [
                mainProductCategoriesNewProvider.overrideWith(() {
                  return mainProductCategoriesNew;
                }),
              ]));

but I got this error:

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building ExploreScreen(dirty, dependencies:
[UncontrolledProviderScope], state: _ExploreScreenState#8fbbe):
Class 'MockMainProductCategoriesNew' has no instance method '_setElement'.
Receiver: Instance of 'MockMainProductCategoriesNew'
Tried calling: _setElement(Instance of 'AutoDisposeNotifierProviderElement<MainProductCategoriesNew,
UIState<List<MainProductCategoryModel>>>')

I also have used mockito :

class MockMainProductCategoriesNew extends Mock
    implements MainProductCategoriesNew {
  @override
  UIState<List<MainProductCategoryModel>> build() {
    return const UIState.initial();
  }
}

I got this error :

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following _TypeError was thrown running a test:
type 'Null' is not a subtype of type 'Future<void>'

this is my widget

  @override
  void initState() {
    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      ref.read(mainProductCategoriesNewProvider.notifier).getMainProductCategories();
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    var mainProductCategoryState = ref.watch(mainProductCategoriesNewProvider);