[gnome-shell] windowPreview: Handle case where window already is part of layout

[Florian Müllner <fmuellner src gnome org>]

commit 0b2430128a4d34bcb0e8a5d79a731ce05c4e084c
Author: Jonas Dreßler <verdre v0yd nl>
Date:   Sat Oct 24 20:51:56 2020 +0200

    windowPreview: Handle case where window already is part of layout
    
    We sometimes add dialogs multiple times to the WindowPreview, for
    example for modal dialogs we receive both the "window-added" and the
    "window-entered-monitor" signal, which means we call
    WindowPreview.addDialog() twice.
    
    We handle that fine already in the WindowPreviewLayout and return NULL
    in case the window already was added, so simply handle that NULL return
    value and bail out of WindowPreview.addDialog() in this case.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1482

 js/ui/windowPreview.js | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/js/ui/windowPreview.js b/js/ui/windowPreview.js
index 5630ba5e31..d379703dd8 100644
--- a/js/ui/windowPreview.js
+++ b/js/ui/windowPreview.js
@@ -500,6 +500,8 @@ var WindowPreview = GObject.registerClass({
 
     _addWindow(metaWindow) {
         const clone = this._windowContainer.layout_manager.addWindow(metaWindow);
+        if (!clone)
+            return;
 
         // We expect this to be used for all interaction rather than
         // the ClutterClone; as the former is reactive and the latter