Bug 1924058 - Add a null check to prevent broken remote content notification. r=aleca

Differential Revision: https://phabricator.services.mozilla.com/D227035

mail/base/content/msgHdrView.js

@@ -3819,7 +3819,7 @@ var gMessageNotificationBar = {
           aCanOverride ? buttons : []
         )
         .then(notification => {
-          notification.buttonContainer.firstElementChild.classList.add(
+          notification.buttonContainer.firstElementChild?.classList.add(
             "button-menu-list"
           );
         }, console.warn);