C# Export จากกริดเป็น Excel โดยใช้ Class เเล้วเกิดข้อผิดพลาดดังรูปแก้ไขอย่างไรครับ

public void ExportGridView(System.Web.UI.Control gv, System.Web.UI.Page tmpPage)
    {
        PrepareGridViewForExport(gv);
        tmpPage.Response.AddHeader("content-disposition", "attachment;filename=DataReport.xls");

        tmpPage.Response.ContentType = "application/vnd.ms-excel";

        tmpPage.Response.Charset = "";
        tmpPage.EnableViewState = false;
        System.IO.StringWriter tw = new System.IO.StringWriter();
        System.Web.UI.HtmlTextWriter hw = new System.Web.UI.HtmlTextWriter(tw);
        System.Web.UI.HtmlControls.HtmlForm frm = new System.Web.UI.HtmlControls.HtmlForm();
        tmpPage.Controls.Add(frm);
        frm.Controls.Add(gv);
        frm.Attributes["runat"] = "server";
        frm.RenderControl(hw);
        tmpPage.Response.Write(tw.ToString());
        tmpPage.Response.End();
    }
 
    public void PrepareGridViewForExport(System.Web.UI.Control gv)
    {
        LinkButton lb = new LinkButton();
        Literal L = new Literal();
        string name = string.Empty;
       
        for (int i = 0; i <= gv.Controls.Count - 1; i++)
        {
            if (i < gv.Controls.Count)
            {
                if (gv.Controls[i].GetType() == typeof(LinkButton))
                {
                    //Use only text
                    L.Text = ((LinkButton)gv.Controls[i]).Text;
                    gv.Controls.Remove(gv.Controls[i]);
                    gv.Controls.AddAt(i, L);
                }
                else if (gv.Controls[i].GetType() == typeof(DropDownList))
                {
                    //Use only Text
                    L.Text = ((DropDownList)gv.Controls[i]).SelectedItem.Text;
                    gv.Controls.Remove(gv.Controls[i]);
                    gv.Controls.AddAt(i, L);
                }
                else if (gv.Controls[i].GetType() == typeof(CheckBox))
                {
                    //Use only true of false
                    L.Text = (((CheckBox)gv.Controls[i]).Checked ? "True" : "False");
                    gv.Controls.Remove(gv.Controls[i]);
                    gv.Controls.AddAt(i, L);
                }
            }

            if (i < gv.Controls.Count)
            {
                if ((gv.Controls[i].HasControls()))
                {
                    //if remain control
                    PrepareGridViewForExport(gv.Controls[i]);
                }
            }
        }
    }