[UITests] fix dismiss page crash (#4251)

Xamarin.Forms.Controls/CoreGalleryPages/CoreGalleryPage.cs

@@ -48,7 +48,13 @@ namespace Xamarin.Forms.Controls
 
 			var modalDismissButton = new Button () {
 				Text = "Dismiss Page",
-				Command = new Command (async () => await Navigation.PopModalAsync ())
+				Command = new Command (async () =>
+				{
+					if (_picker.SelectedIndex == 0)
+						await Navigation.PopAsync();
+					else
+						_picker.SelectedIndex--;
+				})
 			};
 			Layout.Children.Add (modalDismissButton);