Fatal error: Uncaught Error: Class "Drupal\Core\Form\FormBase" not found in /home/www/drupal8/test/core/modules/system/src/Form/ModulesUninstallForm.php:17 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/system/src/Form/ModulesUninstallForm.php on line 17