could not find repository

XF\Mvc\Entity\Manager->getRepository() in src/XF/App.php at line 2771 XF\App->repository() in src/XF/Validator/Username.php at line 100 XF\Validator\Username->isValid() in src/XF/Entity/User.php at line 1364 XF\Entity\User->verifyUsername() in src/XF/Mvc/Entity/Entity.php at line 791...