Flutter how to mock function response when it is called in tested function

I am trying to test some api call function:

  Future<SomeType> getType(int id) async {
    String token = await retrieveToken();
    String api = await getApiEndpoint();
    final response = await client.get("$api/something/$id", headers: { "Authorization": "Bearer $token"});
    if (response.statusCode == 200) {
      String body = utf8.decode(response.bodyBytes);
      return SomeType.fromJson(json.decode(body));
    } else {
      throw Exception('Failed to load some types from API');
    }
  }

The two functions:

retrieveToken();
getApiEndpoint();

that are called inside are not members of the same class as getType() function. They also contain inside them instances of FlutterSecureStorage().

I don't need to test this function, I just want to mock their responses in the test of getType function.

I've created following test example:

class MockedApiEndpoint extends Mock implements ApiEndpoint {}
void main() {
    WidgetsFlutterBinding.ensureInitialized();
    test('returns SomeType', () async {
      final typesRepository = TypesRepository();
      final mockedApiEndpoint = MockedApiEndpoint();

      when(mockedApiEndpoint.getApiEndpoint()).thenAnswer((_) async => Future.value("some_url.com"));
      when(mockedApiEndpoint.retrieveToken()).thenAnswer((_) async => Future.value("some_token"));

      typesRepository.client = MockClient( (request) async {
        final mapJson = {'id' : 123};
        return Response(json.encode(mapJson), 200);
      });

      final type =  await typesRepository.getType(123);
      expect(type.id, 123);
    });
}

I was able to succesfully Mock the client reposne, I am also trying to mock responses of those two functions, but I am constantly receiveing following error:

MissingPluginException(No implementation found for method read on channel plugins.it_nomads.com/flutter_secure_storage)

I would be very greatful for any hints how to approach this topic and what I am doing wrong with this one. I am new to unit testing in Flutter, I've tried to find any similar problem to mine on the web but unfortunatelly failed.