Fatal error: Uncaught Error: Class "Drupal\Core\Form\ConfirmFormBase" not found in /home/www/drupal8/test/core/modules/search/src/Form/ReindexConfirm.php:14 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/search/src/Form/ReindexConfirm.php on line 14