Issue #3133113 by hatuhay: validate id for menu block

bootstrap_barrio.theme

@@ -348,7 +348,7 @@ function bootstrap_barrio_preprocess_region(&$variables) {
  * Implements hook_preprocess_HOOK() for block.html.twig.
  */
 function bootstrap_barrio_preprocess_block(&$variables) {
-  if ($variables['base_plugin_id'] == 'system_menu_block') {
+  if ($variables['base_plugin_id'] == 'system_menu_block' && isset($variables['attributes']['id'])) {
     $variables['content']['#attributes']['block'] = $variables['attributes']['id'];
   }
   // Add a clearfix class to system branding blocks.