LibGUI: Do not attempt to close non-existent notifications

Author: https://github.com/implicitfield
Commit: ba8bd2e558
Pull-request: https://github.com/SerenityOS/serenity/pull/17852

Userland/Libraries/LibGUI/Notification.cpp

@@ -22,6 +22,7 @@ class ConnectionToNotificationServer final
 public:
     virtual void die() override
     {
+        if (!m_notification->m_destroyed)
             m_notification->connection_closed();
     }