Fatal error: Uncaught Error: Class "Drupal\Core\Plugin\DefaultPluginManager" not found in /home/www/drupal8/test/modules/webform/src/Plugin/WebformExporterManager.php:20 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/webform/src/Plugin/WebformExporterManager.php on line 20