Export to Excel in ASP.Net with C#

HttpException was unhandled by user code

System.Web.HttpException was unhandled by user code
  Message="DataGrid with id '' could not automatically generate any columns from the selected data source."

 protected void Button1_Click(object sender, EventArgs e)
 {
     connectionstring();
     SqlConnection con = new SqlConnection(str);
     con.Open();
     SqlCommand cmd = new SqlCommand("SP_Excel", con);
     cmd.CommandType = CommandType.StoredProcedure;
     cmd.Parameters.Add("@ExcelName", SqlDbType.VarChar).Value = FileUpload1.FileName.ToString();
     cmd.ExecuteNonQuery();
     con.Close();
     Response.Write("<script>alert('Table Imported Successfully');</script>");
     Button1.Visible = false;
     Button2.Visible = true;

     SqlDataAdapter da = new SqlDataAdapter(cmd);
     DataTable dt = new DataTable();
     da.Fill(dt);
     GridView1.DataSource = dt;
     GridView1.DataBind();
     ViewState["dtList"] = dt;


protected void Button2_Click(object sender, EventArgs e)
 {
     DataTable dt1 = (DataTable)ViewState["dtList"];
     if (dt1 == null)
     {
         throw new Exception("No Records to Export");
     }
     string Path = "D:\\ImportExcelFromDatabase\\myexcelfile_" + DateTime.Now.Day.ToString() + "_" + DateTime.Now.Month.ToString() + ".xlsb";
     FileInfo FI = new FileInfo(Path);
     StringWriter stringWriter = new StringWriter();
     HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWriter);
     DataGrid DataGrd = new DataGrid();
     DataGrd.DataSource = dt1;
     DataGrd.DataBind();
     DataGrd.RenderControl(htmlWrite);
     string directory = Path.Substring(0, Path.LastIndexOf("\\"));// GetDirectory(Path);
     if (!Directory.Exists(directory))
     {
         Directory.CreateDirectory(directory);
     }
     System.IO.StreamWriter vw = new System.IO.StreamWriter(Path, true);
     stringWriter.ToString().Normalize();
     vw.Write(stringWriter.ToString());
     vw.Flush();
     vw.Close();
     WriteAttachment(FI.Name, "application/vnd.ms-excel", stringWriter.ToString());
 }
 public static void WriteAttachment(string FileName, string FileType, string content)
 {
     HttpResponse Response = System.Web.HttpContext.Current.Response;
     Response.ClearHeaders();
     Response.AppendHeader("Content-Disposition", "attachment; filename=" + FileName);
     Response.ContentType = FileType;
     Response.Write(content);
     Response.End();
 }