Flutter testing : when testing server failure returns null - flutter

Question

I'm using flutter test to do unit testing. The package use to mocking is mocktail package (https://pub.dev/packages/mocktail). When I run the test which is to test the server failure scenario, the actual result I get from this is null.
But the expected result is <Instance of 'Failure'>.
How can I fix this issue?
import 'package:flutter_test/flutter_test.dart';
import 'package:mocktail/mocktail.dart';
class MockHttpService extends Mock implements HttpService {}
class MockAthleteEnv extends Mock implements AthleteEnv {}
class MockSharedPrefsService extends Mock implements SharedPrefsService {}
class MockAthleteContactDetailsReq extends Mock implements AthleteContactDetailsReq{}
void main() {
late ContactDetailsService contactDetailsService;
late MockHttpService mockHttpService;
late MockAthleteEnv mockAthleteEnv;
late MockSharedPrefsService mockStorageService;
late MockAthleteContactDetailsReq mockAthleteContactDetailsReq;
setUp(() {
mockHttpService = MockHttpService();
mockAthleteEnv = MockAthleteEnv();
mockStorageService = MockSharedPrefsService();
mockAthleteContactDetailsReq = MockAthleteContactDetailsReq();
contactDetailsService = ContactDetailsService(
httpService: mockHttpService,
athleteEnv: mockAthleteEnv,
storageService: mockStorageService);
});
group('add athlete contact details', () {
const tAthleteId = 001;
const tAthleteEndpoint = 'https://endpoint/';
const tData = {
'firstName':'John',
'lastName':'Doe',
'contactNumber': '0932445178',
'areaCode':'94',
'email':'johndoe3#mailinator.com',
};
test(
'should return server failure when the call to remote data source is unsuccessful',
() async {
// Arrange
when(() => mockStorageService.getFromDisk(any()))
.thenAnswer((_) => tAthleteId);
when(() => mockAthleteEnv.API_ATHLETE_ENDPOINT)
.thenAnswer((_) => tAthleteEndpoint);
when(() => mockAthleteContactDetailsReq.toJson()).thenAnswer((_) => tData );
when(() => mockHttpService.post(any(), any())).thenThrow(
DioError(
response: Response(
statusCode: 400,
requestOptions: RequestOptions(path: ""),
),
requestOptions: RequestOptions(path: ""),
)
);
// Act
final result = await contactDetailsService.addAthleteContactDetails(mockAthleteContactDetailsReq);
// Assert
expect(result, throwsA(const TypeMatcher<Failure>()));
});
});
}