Fatal error: Uncaught Error: Class "Drupal\Core\Field\Plugin\Field\FieldWidget\StringTextfieldWidget" not found in /home/www/drupal8/test/core/modules/text/src/Plugin/Field/FieldWidget/TextfieldWidget.php:21 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/text/src/Plugin/Field/FieldWidget/TextfieldWidget.php on line 21