workspacesDisplay: Disconnect MetaLater and parent signals on destroy

When the WorkspacesDisplay actor is destroyed we should remove the
ongoing later and parent widget connections to avoid accessing an
invalid object on callback.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/700

js/ui/workspacesView.js

@@ -524,6 +524,22 @@ var WorkspacesDisplay = class {
         this._keyPressEventId = 0;
 
         this._fullGeometry = null;
+
+        this.actor.connect('destroy', this._onDestroy.bind(this));
+    }
+
+    _onDestroy() {
+        if (this._notifyOpacityId) {
+            let parent = this.actor.get_parent();
+            if (parent)
+                parent.disconnect(this._notifyOpacityId);
+            this._notifyOpacityId = 0;
+        }
+
+        if (this._parentSetLater) {
+            Meta.later_remove(this._parentSetLater);
+            this._parentSetLater = 0;
+        }
     }
 
     _onPan(action) {
@@ -717,7 +733,11 @@ var WorkspacesDisplay = class {
             oldParent.disconnect(this._notifyOpacityId);
         this._notifyOpacityId = 0;
 
-        Meta.later_add(Meta.LaterType.BEFORE_REDRAW, () => {
+        if (this._parentSetLater)
+            return;
+
+        this._parentSetLater = Meta.later_add(Meta.LaterType.BEFORE_REDRAW, () => {
+            this._parentSetLater = 0;
             let newParent = this.actor.get_parent();
             if (!newParent)
                 return;