public function addValue(string $value, ?string $usergroup = null): void
    {
        if (array_key_exists($usergroup, $this->getAllValues())) {
            throw new DuplicateValueForUsergroupException($this->getKey(), $usergroup);
        }

        $this->values[$usergroup] = DataHelper::checkForEmptyValue('propertyValue', $value);
    }