asp.net+nopi生成Excel遇到设置单元格值null问题

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("新闻");
sheet1.GetRow(2).GetCell(1).SetCellValue("娱乐");
sheet1.GetRow(3).GetCell(1).SetCellValue("Angel工作室");
//Force excel to recalculate all the formula while open
sheet1.ForceFormulaRecalculation = true;
//输出报表
HssfworkbookToaDownloadFile(@"测试表名称.xls", hssfworkbooks); /// <summary>
/// hssfworkbook输出为下载文件
/// </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();
}

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