src/FormatMapping.php
private function filterString($str): string
    {
        if (is_string($str) && strlen($str) > 0 && $str !== null) {
            return $str;
        }
        throw new \Exception('String required', 1);
    }
private function filterString($str): string
    {
        if (is_string($str) && strlen($str) > 0 && $str !== null) {
            return $str;
        }
        throw new \Exception('String required', 1);
    }
Make sure string is not null or empty
@param   mixed $str
@return string if it is valid or exception
@throws \Exception