panelMenu: Destroy menu before chaining up

This avoid some (harmless but annoying) warnings, and is closer to
the original code prior to commit fc342fe8c5 and 557b232c896.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3022

js/ui/panelMenu.js

@@ -183,10 +183,9 @@ var Button = GObject.registerClass({
     }
 
     _onDestroy() {
-        super._onDestroy();
-
         if (this.menu)
             this.menu.destroy();
+        super._onDestroy();
     }
 });