Fatal error: Uncaught Error: Class "Drupal\Core\Form\ConfigFormBase" not found in /home/www/drupal8/test/modules/contrib/swiftmailer/src/Form/MessagesForm.php:13 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/contrib/swiftmailer/src/Form/MessagesForm.php on line 13