Bug 1865806 - Prevent blank 3pane when starting up with unsent messages in outbox. r=freaktechnik

Differential Revision: https://phabricator.services.mozilla.com/D194211

--HG--
extra : moz-landing-system : lando

mail/base/content/about3Pane.js

@@ -4322,7 +4322,7 @@ var threadPane = {
   cardColumns: getDefaultColumnsForCardsView(gFolder),
 
   async init() {
-    quickFilterBar.init();
+    await quickFilterBar.init();
 
     this.setUpTagStyles();
     Services.prefs.addObserver("mailnews.tags.", this);

mail/base/content/quickFilterBar.js

@@ -56,7 +56,8 @@ var quickFilterBar = {
    */
   activeElement: null,
 
-  init() {
+  async init() {
+    await window.customElements.whenDefined("search-bar");
     this._bindUI();
     this.updateRovingTab();