[gnome-shell] workspace: Check if index is valid before using it

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

commit 35fb221a7ea99cbc0a7f50782c2b52ab461a21ab
Author: Ivan Molodetskikh <yalterz gmail com>
Date:   Sat Feb 27 09:59:21 2021 +0300

    workspace: Check if index is valid before using it
    
    If the window wasn't in _windowSlots, the index is -1, so the last
    element of the array is removed, leading to
    https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3547
    
    I logged the stack trace from removeWindow() and it seems that when you
    move the window from the second monitor to another workspace like shown
    in https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3547,
    removeWindow() is called twice for it, presumably once for the second
    monitor workspace, and then once for the first monitor workspace for
    some reason. It is during that call that _windowSlots doesn't contain
    the window, so instead the last element (index -1) is removed, leading
    to the animation bug.
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1727>

 js/ui/workspace.js | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/js/ui/workspace.js b/js/ui/workspace.js
index a21b5b43ca..4fa113bac3 100644
--- a/js/ui/workspace.js
+++ b/js/ui/workspace.js
@@ -784,8 +784,9 @@ var WorkspaceLayout = GObject.registerClass({
 
         // The layout might be frozen and we might not update the windowSlots
         // on the next allocation, so remove the slot now already
-        this._windowSlots.splice(
-            this._windowSlots.findIndex(s => s[4] === window), 1);
+        const index = this._windowSlots.findIndex(s => s[4] === window);
+        if (index !== -1)
+            this._windowSlots.splice(index, 1);
 
         // The window might have been reparented by DND
         if (window.get_parent() === this._container)