app Application.Current.MainPage = new NavigationPage(new Pages()); not working on XF 4.0

I have a problem guys, I have now on many places in my app Application.Current.MainPage = new NavigationPage(new Pages()); and with Xamarin Forms 4.0 this doesnt work and crashed the app, how can I temporary work around this? I do want to implement the new navigation but for now I have to fix this. PLease help.