private static function validSheetObj($sheetObj=NULL)
    {
        if (is_object($sheetObj)) {

            return $sheetObj;
        } 
        elseif (is_object(self::$_objSheet)) {

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

            // Set to default sheet if is unset
            return self::setSheet()->getSheet();
        }
        else {
            
            throw new Exception("Invalid or empty PhpSpreadsheet Sheet Object", 400);
        }
    }