overviewControls: Don't hide destroyed child widget on unmap

When we unmap, the child widgets have already been destroyed, so we
shouldn't try to. To detect this, delete the references we keep to them
on destroy, and null-check the hide call.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2623>

js/ui/overviewControls.js

@@ -506,6 +506,8 @@ class ControlsManager extends St.Widget {
             () => this._shiftState(Meta.MotionDirection.DOWN));
 
         this._update();
+
+        this.connect('destroy', this._onDestroy.bind(this));
     }
 
     _getFitModeForState(state) {
@@ -715,7 +717,16 @@ class ControlsManager extends St.Widget {
 
     vfunc_unmap() {
         super.vfunc_unmap();
-        this._workspacesDisplay.hide();
+        this._workspacesDisplay?.hide();
+    }
+
+    _onDestroy() {
+        delete this._searchEntryBin;
+        delete this._appDisplay;
+        delete this.dash;
+        delete this._searchController;
+        delete this._thumbnailsBox;
+        delete this._workspacesDisplay;
     }
 
     prepareToEnterOverview() {