Mockito: Unable to add data to stream; when i call stream sink function it throws function call on null

Unable to add data to stream while testing using mockito. When I call stream sink function then it throws function call on null. The auth.changeShowProgress(true) return function call on null. How can I add data to the stream in the bloc?

testWidgets('Correct email and password gives success',
  (WidgetTester widgetTester) async {
final AuthBloc auth = MockAuthBloc();

when(auth.showProgress).thenReturn(null);

when(auth.changeShowProgress(true)).thenAnswer((_) {
  Future.value(null);
});

await widgetTester.pumpWidget(
  makeTestableWidget(
    authbloc: auth,
    child: MaterialApp(home: LoginPage()),
  ),
);

await widgetTester.pump(Duration.zero);

verify(auth.showProgress).called(1);

expect(find.byType(CircularProgressIndicator), findsOneWidget);

await widgetTester.enterText(
    find.bySemanticsLabel("Email address"), "admin@admin.com");

await widgetTester.enterText(find.bySemanticsLabel("Password"), "password");

await widgetTester.tap(find.widgetWithText(PRaisedButton, "Login"));

await widgetTester.pump();
});