Bug 1848601 - Attempt to fix "can't access property "getBoundingClientRect" in browser_edit_async.js. r=mkmelin

This is based on the fix for the same issue accepted in D207481.

Differential Revision: https://phabricator.services.mozilla.com/D207526

--HG--
extra : amend_source : 9a9232da1f802f03aaa0588d9f07be9abfa14376

mail/components/addrbook/test/browser/browser_edit_async.js

@@ -260,8 +260,10 @@ add_task(async function testModificationUpdatesUI() {
   Assert.equal(cardsList.view.rowCount, 1);
 
   // Display a card.
-
-  EventUtils.synthesizeMouseAtCenter(cardsList.getRowAtIndex(0), {}, abWindow);
+  const row0 = await TestUtils.waitForCondition(() =>
+    cardsList.getRowAtIndex(0)
+  );
+  EventUtils.synthesizeMouseAtCenter(row0, {}, abWindow);
   await TestUtils.waitForCondition(() =>
     BrowserTestUtils.isVisible(detailsPane)
   );