Fatal error: Uncaught Error: Class "Drupal\Core\Form\FormBase" not found in /home/www/drupal8/test/modules/contrib/htmlmail/src/Form/HtmlMailTestForm.php:18 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/contrib/htmlmail/src/Form/HtmlMailTestForm.php on line 18