Fatal error: Uncaught Error: Class "Drupal\webform\Plugin\WebformExporter\DocumentBaseWebformExporter" not found in /home/www/drupal8/test/modules/webform/src/Plugin/WebformExporter/YamlWebformExporter.php:21 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/webform/src/Plugin/WebformExporter/YamlWebformExporter.php on line 21