Fatal error: Uncaught Error: Class "Drupal\Core\Form\ConfirmFormBase" not found in /home/www/drupal8/test/core/modules/book/src/Form/BookRemoveForm.php:16 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/book/src/Form/BookRemoveForm.php on line 16