Fatal error: Uncaught Error: Class "Drupal\Core\Form\FormBase" not found in /home/www/drupal8/test/modules/webform/src/Form/WebformHelpVideoForm.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/webform/src/Form/WebformHelpVideoForm.php on line 15