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