Dart: type ‘Null’ is not a subtype of type ‘Future’ in Mockito

Your signature for MethodChannel.invokeMethod function says it returns a Future<String?> but since the MockMethodChannel doesnot have any implementation for invokeMethod() , it will return null; dart’s null safety gets angry because you lied. For a quick fix, the invokeMethod can have a return type of Future<String?>?. When you do so, you are saying null safety shouldn’t bother even if the returned value is null.

However this is not a permanent solution I just want you to understand the problem.

you could add build_runner in your dev_dependencies: in pubspec.yaml

dart pub add build_runner --dev

and modify your code to

import 'package:flutter/services.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/mockito.dart';
//modified
import 'package:mockito/annotations.dart';
import 'generated file.dart';

class MockMethodChannel extends Mock implements MethodChannel {}

//modified
@GenerateMocks([MockMethodChannel])
void main() {

  group('all', () {

      test('test', () async {
        final mockMethodChannel = MockMockMethodChannel();
        when(mockMethodChannel
            .invokeMethod<String>("GET com.eight/preference-management/preferences"))
            .thenAnswer((_) async => "test");
      });
  });
}

and run
flutter pub run build_runner build --delete-conflicting-outputs for build runner to build the stub files for you