this is much better, thanks! But I still have one or two issues:
1) I now removed all GetViewModelType() methods and the viewLocator.Register after switching my code in the App() to
var serviceLocator = ServiceLocator.Instance;
var viewModelLocator = ServiceLocator.Instance.ResolveType<IViewModelLocator>();
var viewLocator = ServiceLocator.Instance.ResolveType<IViewLocator>();
this works except for the MainWindow, where I still need the GetViewModelType().
2) I also tried the [UP] syntax like this:
But this is not working. It's not really an issue though, as putting the full type at one place is acceptable, it would have just been a bit more clear/nice.