WindowServer: Stop trying to close windows that are already destroyed

Author: https://github.com/AtkinsSJ
Commit: fd7163b125
Pull-request: https://github.com/SerenityOS/serenity/pull/10983
Issue: https://github.com/SerenityOS/serenity/issues/10982

Userland/Services/WindowServer/Window.cpp

@@ -898,6 +898,9 @@ void Window::window_menu_activate_default()
 
 void Window::request_close()
 {
+    if (is_destroyed())
+        return;
+
     Event close_request(Event::WindowCloseRequest);
     event(close_request);
 }