Widgets: Avoid a TypeError when adding a widget in the Customizer.

This fixes a bug where `action` is set to `null` when adding a widget in the Customizer.

Fixes #53488. See #53421, #53419.

git-svn-id: https://develop.svn.wordpress.org/trunk@51238 602fd350-edb4-49c9-b593-d223f7449a82

src/js/_enqueues/wp/util.js

@@ -120,6 +120,8 @@ window.wp = window.wp || {};
 						var context = this;
 						deferred.done( function() {
 							if (
+								action &&
+								action.data &&
 								'query-attachments' === action.data.action &&
 								deferred.jqXHR.hasOwnProperty( 'getResponseHeader' ) &&
 								deferred.jqXHR.getResponseHeader( 'X-WP-Total' )