Customizer: Prevent JS error during init when nav_menus panel is removed by plugin.

Cherry-picks [33753] onto 4.3 branch.
Fixes #33411 for 4.3.


git-svn-id: https://develop.svn.wordpress.org/branches/4.3@33943 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/js/customize-nav-menus.js

@@ -119,6 +119,10 @@
 		initialize: function() {
 			var self = this;
 
+			if ( ! api.panel.has( 'nav_menus' ) ) {
+				return;
+			}
+
 			this.$search = $( '#menu-items-search' );
 			this.sectionContent = this.$el.find( '.accordion-section-content' );