Neos\Neos\Validation\Validator\UserDoesNotExistValidator::isValid PHP Метод

    protected function isValid($value)
    {
        if (!is_string($value)) {
            throw new InvalidSubjectException('The given username was not a string.', 1325155784);
        }
        if ($this->userService->getUser($value) !== null) {
            $this->addError('The username is already in use.', 1325156008);
        }
    }