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

@@ -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