Fix crash when ItemsSource is set to null (#320)

Xamarin.Forms.Platform.Android/AppCompat/TabbedPageRenderer.cs

@@ -190,7 +190,8 @@ namespace Xamarin.Forms.Platform.Android.AppCompat
 			base.OnElementPropertyChanged(sender, e);
 
 			if (e.PropertyName == nameof(TabbedPage.CurrentPage))
-				ScrollToCurrentPage();
+				if(Element.CurrentPage != null)
+					ScrollToCurrentPage();
 			else if (e.PropertyName == NavigationPage.BarBackgroundColorProperty.PropertyName)
 				UpdateBarBackgroundColor();
 			else if (e.PropertyName == NavigationPage.BarTextColorProperty.PropertyName)