code: public function validate($value, Constraint $constraint)
      {
          if (null === $value) {
              return;
          }
  
  
          if (!$this->registry->has($value)) {
              $this->context->buildViolation($constraint->message)
                  ->setParameter('{{string}}', $value)
                  ->addViolation();
          }
      }
docs: {@inheritdoc}