private static function validExcelObj($excelObj=NULL)
    {
        if (is_object($excelObj)) {

            return $excelObj;
        } 
        elseif (is_object(self::$_objSpreadsheet)) {

            return self::$_objSpreadsheet;
        } 
        else {
            
            throw new Exception("Invalid or empty PhpSpreadsheet Object", 400);
        }
    }