Fix a crash when closing the app when running (#269)

Author: mattleibow

source/SkiaSharp.Extended.UI/Controls/SKAnimatedSurfaceView.shared.cs

@@ -82,6 +82,14 @@ private void UpdateIsAnimationEnabled()
 				if (!weakThis.TryGetTarget(out var strongThis))
 					return false;
 
+#if !XAMARIN_FORMS
+				if (strongThis.Window is null)
+					return false;
+#endif
+
+				if (!strongThis.IsLoadedEx())
+					return false;
+
 				strongThis.Invalidate();
 
 				return strongThis.IsAnimationEnabled;