Fatal error: Uncaught Error: Class "Drupal\Core\Field\Plugin\Field\FieldType\StringItem" not found in /home/www/drupal8/test/core/modules/user/src/UserNameItem.php:11 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/user/src/UserNameItem.php on line 11