Bug 1956952 - Add a probe for when CPM is enabled in sidebar customization panel. r=rsafaeian,sidebar-reviewers,kcochrane

Differential Revision: https://phabricator.services.mozilla.com/D243389

browser/components/sidebar/browser-sidebar.js

@@ -627,7 +627,8 @@ var SidebarController = {
     let observer = this._observer;
     if (!observer) {
       observer = new MutationObserver(() => {
-        this.title = this.sidebars.get(this.lastOpenedId).title;
+        // it's possible for lastOpenedId to be null here
+        this.title = this.sidebars.get(this.lastOpenedId)?.title;
       });
       // Re-use the observer.
       this._observer = observer;