Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Nullability of reference types in return type of 'object? ContentControl.Content.get' doesn't match implicitly implemented member 'object INotificationMessageButton.Content.get' (possibly because of nullability attributes).

Nullability of reference types in return type of 'IBrush? TemplatedControl.Foreground.get' doesn't match implicitly implemented member 'IBrush INotificationMessage.Foreground.get' (possibly because of nullability attributes).

Nullability of reference types in return type of 'IBrush? TemplatedControl.Background.get' doesn't match implicitly implemented member 'IBrush INotificationMessage.Background.get' (possibly because of nullability attributes).

Nullability of reference types in type of parameter 'parameter' of 'bool CloseCommand.CanExecute(object parameter)' doesn't match implicitly implemented member 'bool ICommand.CanExecute(object? parameter)' (possibly because of nullability attributes).

Nullability of reference types in type of parameter 'parameter' of 'void CloseCommand.Execute(object parameter)' doesn't match implicitly implemented member 'void ICommand.Execute(object? parameter)' (possibly because of nullability attributes).

Cannot convert null literal to non-nullable reference type.

Non-nullable property 'Callback' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.