[gnome-shell] workspaceAnimation: Fix warning on restacking

[Marge Bot <marge-bot src gnome org>]

commit 5d1b9a429e11260fed2d0064629452a6bcda73ad
Author: Florian Müllner <fmuellner gnome org>
Date:   Tue Feb 22 21:16:52 2022 +0100

    workspaceAnimation: Fix warning on restacking
    
    Only workspace groups that are associated with a workspace have a
    background. As a result, when restacking window previews we end up
    passing `undefined` to clutter_actor_set_child_above_sibling()
    instead of null, triggering a warning.
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2209>

 js/ui/workspaceAnimation.js | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/js/ui/workspaceAnimation.js b/js/ui/workspaceAnimation.js
index f61afe323b..25144ab3ac 100644
--- a/js/ui/workspaceAnimation.js
+++ b/js/ui/workspaceAnimation.js
@@ -73,11 +73,13 @@ class WorkspaceGroup extends Clutter.Actor {
             this._shouldShowWindow(w.meta_window));
 
         let lastRecord;
+        const bottomActor = this._background ?? null;
 
         for (const windowActor of windowActors) {
             const record = this._windowRecords.find(r => r.windowActor === windowActor);
 
-            this.set_child_above_sibling(record.clone, lastRecord ? lastRecord.clone : this._background);
+            this.set_child_above_sibling(record.clone,
+                lastRecord ? lastRecord.clone : bottomActor);
             lastRecord = record;
         }
     }