public function getListId(string $listId = null): ?string
    {
        return ($listId == null) ? $this->listId : $listId;
    }
public function getListId(string $listId = null): ?string
    {
        return ($listId == null) ? $this->listId : $listId;
    }
public function getListId ( string $ listId = null ) : ? string { return ( $ listId == null ) ? $ this -> listId : $ listId ; }
Get list id

@param string|null $listId - If you want to use a custom list id
@return string|null
Get list id