Fatal error: Uncaught Error: Class "Drupal\Core\Render\Element\RenderElement" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Render/Element/Dropbutton.php:42 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Render/Element/Dropbutton.php on line 42