Fatal error: Uncaught Error: Class "Drupal\Core\Render\Element\FormElement" not found in /home/www/drupal8/test/modules/webform/src/Element/WebformElementMultiple.php:18
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/modules/webform/src/Element/WebformElementMultiple.php on line 18