[WIP] Fix for InvalidOperationException when iOS app starts

[chrisstaley]

The Exception occurs when the iOS app launches. It is due to NavigationPage being rendered before the root view has been set. Android applications are not affected since the root page check occurs in platform-specific code.

Exception details are:

NavigationPage must have a root Page before being used. Either call PushAsync with a valid Page, or pass a Page to the constructor before usage. (System.InvalidOperationException)
at Xamarin.Forms.Platform.iOS.NavigationRenderer.ViewDidLoad () <0x10115fc60 + 0x0071c> in <bc5a3d4303744676b3b36f4254bb697d#fcf3a1c8209da1af9ec91eac76a1af7b>:0
at (wrapper managed-to-native) ObjCRuntime.Messaging:objc_msgSendSuper (intptr,intptr)
at UIKit.UIViewController.get_View () <0x100a62790 + 0x00073> in <e0596b82250c450abdf075bc558add28#fcf3a1c8209da1af9ec91eac76a1af7b>:0
at Xamarin.Forms.Platform.iOS.NavigationRenderer.get_NativeView () <0x10115ef90 + 0x0001b> in <bc5a3d4303744676b3b36f4254bb697d#fcf3a1c8209da1af9ec91eac76a1af7b>:0
at Xamarin.Forms.Platform.iOS.NavigationRenderer.SetElement (Xamarin.Forms.VisualElement element) <0x10115efc0 + 0x000bb> in <bc5a3d4303744676b3b36f4254bb697d#fcf3a1c8209da1af9ec91eac76a1af7b>:0
at Xamarin.Forms.Platform.iOS.Platform.CreateRen<\M-b\M^@\M-&>

For iOS, you must wait to set the application's MainPage property until after the NavigationPage's root view has been set (which is handled by the call to navigationService.NavigateToViewAsync()).