It seems that when using DisplayRootViewFor<T>(), the RegisterNavigationService (on PrepareViewFirst) method is not called and you end up with null passed in for the INavigationService.
This didn't happen on the previous version.
Comments: I'm not sure I follow by what you mean by bad manners, the RegisterNavigationService on WinRTContainer isn't called automatically is completely up to you to call. As mentioned in the documentation the WinRT version is the first that explicitly supports both a view first and view model first approach, previously the Windows Phone version was view first while other platforms were view model first. There's nothing stopping you using a view model first approach while having an internal Frame somewhere and manually registering it as a navigation service.
This didn't happen on the previous version.
Comments: I'm not sure I follow by what you mean by bad manners, the RegisterNavigationService on WinRTContainer isn't called automatically is completely up to you to call. As mentioned in the documentation the WinRT version is the first that explicitly supports both a view first and view model first approach, previously the Windows Phone version was view first while other platforms were view model first. There's nothing stopping you using a view model first approach while having an internal Frame somewhere and manually registering it as a navigation service.