Fatal error: Uncaught Error: Class "Drupal\Core\Plugin\PluginBase" not found in /home/www/drupal8/test/modules/webform/src/Plugin/WebformSourceEntity/QueryStringWebformSourceEntity.php:26 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/webform/src/Plugin/WebformSourceEntity/QueryStringWebformSourceEntity.php on line 26