Fatal error: Uncaught Error: Interface "Drupal\Core\Form\FormInterface" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Form/ConfirmFormInterface.php:8 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Form/ConfirmFormInterface.php on line 8