Bug 1933761 - Fix view sidebar menu checked states r=sidebar-reviewers,sfoster

Any menu items that were added after `viewCustomizeSidebar` in `this.sidebars`, such as extensions, were previous ignored in `selectMenuItem` thus resulting in an incorrect checked state in the menu.

Differential Revision: https://phabricator.services.mozilla.com/D232648

browser/components/sidebar/browser-sidebar.js

@@ -1668,7 +1668,7 @@ var SidebarController = {
     for (let [id, { menuId, triggerButtonId }] of this.sidebars) {
       let menu = document.getElementById(menuId);
       if (!menu) {
-        return;
+        continue;
       }
       let triggerbutton =
         triggerButtonId && document.getElementById(triggerButtonId);