window-list: Handle no overrides settings

We can only know about override settings that are provided by the
upstream GNOME or GNOME Classic sessions, but not any custom sessions
created by admins, users or distributions. Handle that case by falling
back to the original settings.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/issues/62

extensions/window-list/extension.js

@@ -888,7 +888,7 @@ class WindowList {
     }
 
     _getWorkspaceSettings() {
-        let settings = global.get_overrides_settings();
+        let settings = global.get_overrides_settings() || this._mutterSettings;
         if (settings.list_keys().indexOf('workspaces-only-on-primary') > -1)
             return settings;
         return this._mutterSettings;