Fatal error: Uncaught Error: Class "Drupal\Core\Form\FormBase" not found in /home/www/drupal8/test/core/modules/config/src/Form/ConfigSingleExportForm.php:21 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/config/src/Form/ConfigSingleExportForm.php on line 21