Ticket #33974: 33974.diff

src/wp-includes/post-template.php

@@ -1275 +1275 @@
 
         $container = sanitize_text_field( $args['container'] );
 
+        // Fallback in case `wp_nav_menu()` was called without a container.
+        if ( empty( $container ) ) {
+                $container = 'div';
+        }
+
         if ( $menu ) {
 
                 // wp_nav_menu doesn't set before and after