overviewControls: Fix warning on shutdown

ClutterActor unsets its layout manager's :container property during
dispose, so the corresponding vfunc should handle being passed NULL.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1909>

js/ui/overviewControls.js

@@ -112,6 +112,7 @@ class ControlsManagerLayout extends Clutter.BoxLayout {
 
     vfunc_set_container(container) {
         this._container = container;
+        if (container)
             this.hookup_style(container);
     }