Fatal error: Uncaught Error: Class "Drupal\webform\Plugin\WebformElement\Checkboxes" not found in /home/www/drupal8/test/modules/webform/src/Plugin/WebformElement/WebformEntityCheckboxes.php:17 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/webform/src/Plugin/WebformElement/WebformEntityCheckboxes.php on line 17