Fatal error: Uncaught Error: Class "Drupal\Core\Form\ConfigFormBase" not found in /home/www/drupal8/test/modules/custom/my_mailer/src/Form/MyMailerForm.php:8 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/custom/my_mailer/src/Form/MyMailerForm.php on line 8