Editing Condition "Webform is one of" results in Class 'CRM_Core_Smarty' not found error

It starts with a "network not found" but following the url civicrm/civirules/conditions/webform_civirules_webform_is?rule_condition_id=2&snippet=json it results in a

Error: Class 'CRM_Core_Smarty' not found in webform_civirules_menu_get_item_alter() (line 12 of /home/klaas/work/drupal/sites/all/modules/webform_civirules/webform_civirules.module).