Ticket #34111: 34111.patch

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

@@ -1132 +1132 @@
                                         }
                                 });
                                 if ( settingValue ) {
+                                        if ( property === 'classes' && _.isArray( settingValue[ property ] ) ) {
+                                                settingValue[ property ] = settingValue[ property ].join( " " );
+                                        }
+
                                         element.set( settingValue[ property ] );
                                 }
                         });