Bug 1630192 - Wait for the page to load before attempting to retrieve element. r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D72506

browser/components/customizableui/test/browser_947914_button_addons.js

@@ -5,7 +5,6 @@
 "use strict";
 
 var initialLocation = gBrowser.currentURI.spec;
-var newTab = null;
 
 add_task(async function() {
   CustomizableUI.addWidgetToArea(
@@ -23,10 +22,14 @@ add_task(async function() {
   ok(addonsButton, "Add-ons button exists in Panel Menu");
   addonsButton.click();
 
-  newTab = gBrowser.selectedTab;
-  await TestUtils.waitForCondition(
-    () => gBrowser.currentURI && gBrowser.currentURI.spec == "about:addons"
-  );
+  await Promise.all([
+    TestUtils.waitForCondition(
+      () => gBrowser.currentURI && gBrowser.currentURI.spec == "about:addons"
+    ),
+    new Promise(r =>
+      gBrowser.selectedBrowser.addEventListener("load", r, true)
+    ),
+  ]);
 
   let addonsPage = gBrowser.selectedBrowser.contentWindow.document.getElementById(
     "addons-page"