Bug 1735080 - Avoid throwing an exception and failing test browser_newmailaccount.js. rs=bustage-fix DONTBUILD

--HG--
extra : rebase_source : 38e022568fb03fc8d039f12ab3ce718c527cfe8f
extra : amend_source : 2cf0599bc093c0f4bd73355b51642fd5772a5a68

mail/test/browser/newmailaccount/browser_newmailaccount.js

@@ -256,7 +256,7 @@ add_task(async function test_account_creation_from_provisioner() {
   await BrowserTestUtils.waitForCondition(
     () =>
       mc.tabmail.selectedTab.browser.contentWindow.gAccountSetup
-        ._currentModename == "success",
+        ?._currentModename == "success",
     "The success view was shown"
   );