asp.net+nopi Excel 생성 셀 값 null 설정 문제

        FileStream file = new FileStream(Server.MapPath("Templatetest.xls"), FileMode.Open, FileAccess.Read);

        HSSFWorkbook existWorkbook = new HSSFWorkbook(file);

        HSSFSheet sheet1 = (HSSFSheet)existWorkbook.GetSheet("Sheet1");
         sheet1.GetRow(1).GetCell(1).SetCellValue("  ");
         sheet1.GetRow(2).GetCell(1).SetCellValue("  ");
         sheet1.GetRow(3).GetCell(1).SetCellValue("Angel   ");
         //Force excel to recalculate all the formula while open
         sheet1.ForceFormulaRecalculation = true;
         //    
         HssfworkbookToaDownloadFile(@"     .xls", hssfworkbooks);




    /// <summary>
    /// hssfworkbook       
    /// </summary>
    /// <param name="filename"></param>
    private void HssfworkbookToaDownloadFile(string filename, HSSFWorkbook hssfworkbook)
    {
        if (HttpContext.Current.Request.UserAgent.ToLower().IndexOf("msie") > -1)
        {
            filename = HttpContext.Current.Server.UrlPathEncode(filename);
        }
        HttpContext.Current.Response.ContentType = "application/vnd.ms-excel";
        HttpContext.Current.Response.AddHeader("Content-Disposition", string.Format("attachment;filename={0}", filename));
        HttpContext.Current.Response.Clear();
        MemoryStream file = new MemoryStream();
        hssfworkbook.Write(file);
        HttpContext.Current.Response.BinaryWrite(file.GetBuffer());
        HttpContext.Current.Response.End();
    }

    sheet1.GetRow(1).GetCell(1).SetCellValue("  ");
         null  。         ，                        。
      ：

            if (sheet1.GetRow(1) == null)
            {
                sheet1.CreateRow(1);
            }
            if (sheet1.GetRow(1).GetCell(1) == null)
            {
                sheet1.GetRow(1).CreateCell(1);
            }