Ticket #41488: 41488.diff

src/wp-includes/class-wp-customize-nav-menus.php

@@ -1210 +1210 @@
                         ! empty( $args['echo'] )
                         &&
                         // ...and if the fallback_cb can be serialized to JSON, since it will be included in the placement context data,
-                        ( empty( $args['fallback_cb'] ) || is_string( $args['fallback_cb'] ) )
+                        ( empty( $args['fallback_cb'] ) || ( is_string( $args['fallback_cb'] ) && false === strpos( $args['fallback_cb'], '\\' ) ) )
                         &&
                         // ...and if the walker can also be serialized to JSON, since it will be included in the placement context data as well,
                         ( empty( $args['walker'] ) || is_string( $args['walker'] ) )