Clear PopupFlyout target on flyout close (#15466)

* Clear PopupFlyout target on flyout close

* Clear Target after OnClose is called

src/Avalonia.Controls/Flyouts/PopupFlyoutBase.cs

@@ -205,6 +205,8 @@ namespace Avalonia.Controls.Primitives
 
             OnClosed();
 
+            Target = null;
+
             return true;
         }