[A] Prevent crash in event when renderer is null in Layout call. (#457)

Reproduction unknown, issue deduced from stack traces from HomeAway.

Xamarin.Forms.Platform.Android/Platform.cs

@@ -1020,7 +1020,7 @@ namespace Xamarin.Forms.Platform.Android
 			}
 
 			foreach (IVisualElementRenderer view in _navModel.Roots.Select(GetRenderer))
-				view.UpdateLayout();
+				view?.UpdateLayout();
 		}
 
 		SizeRequest IPlatform.GetNativeSize(VisualElement view, double widthConstraint, double heightConstraint)
@@ -1069,4 +1069,4 @@ namespace Xamarin.Forms.Platform.Android
 
 		#endregion
 	}
-}
+}