Update Growl.cs

Fix Growl is not enabled while window is close before Growl  is closed

src/Shared/HandyControl_Shared/Controls/Growl/Growl.cs

@@ -439,8 +439,19 @@ public class Growl : Control
 
     private static Panel CreateDefaultPanel()
     {
-        FrameworkElement element = WindowHelper.GetActiveWindow();
+       var win = WindowHelper.GetActiveWindow();
-        var decorator = VisualHelper.GetChild<AdornerDecorator>(element);
+        win.Closed+= (s, e) =>
+        {
+            if (GrowlPanel != null)
+            {
+                foreach (var item in GrowlPanel.Children.OfType<Growl>())
+                {
+                    item.Close(false);
+                }
+                GrowlPanel = null;
+            }
+        };
+        var decorator = VisualHelper.GetChild<AdornerDecorator>(win);
 
         var layer = decorator?.AdornerLayer;
         if (layer == null)