ASP.net ติด error ตรง Gridview method OnPrerender ช่วยดูให้หน่อยคะ

    <asp:GridView ID="GridView3" runat="server" AutoGenerateColumns="False" BorderStyle="None" BorderWidth="0px" 
     OnPrerender="GridView3_PreRender">
            <Columns>
            <asp:BoundField HeaderText="ระบบ" DataField="source" >
                <HeaderStyle BackColor="#AD65BC" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" ForeColor="White" />
                <ItemStyle BackColor="#F4DFFF" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" HorizontalAlign="Left" VerticalAlign="Middle" Width="100px" />
                </asp:BoundField>
            <asp:BoundField HeaderText="อุปกรณ์" DataField="equipment">
                <HeaderStyle BackColor="#AD65BC" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" ForeColor="White" />
                <ItemStyle BackColor="#F4DFFF" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" HorizontalAlign="Left" VerticalAlign="Middle" 
                    Width="150px" />
                </asp:BoundField>
            <asp:BoundField HeaderText="ชื่อไฟล์" DataField="filename" >
                <HeaderStyle BackColor="#AD65BC" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" ForeColor="White" />
                <ItemStyle BackColor="#F4DFFF" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" HorizontalAlign="Left" VerticalAlign="Middle" 
                    Width="175px" />
                </asp:BoundField>
            <asp:BoundField HeaderText="ข้อมูล(เดือน/ปี)" DataField="DATA_DATE" 
                    Visible="False" >
                <ControlStyle BorderStyle="Solid" />
                <HeaderStyle BackColor="#AD65BC" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" ForeColor="White" />
                <ItemStyle BackColor="#F4DFFF" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" HorizontalAlign="Center" VerticalAlign="Middle" 
                    Width="100px" />
                </asp:BoundField>
            <asp:BoundField HeaderText="วันที่นำเข้า" DataField="IMPORT_DATE" >
                <HeaderStyle BackColor="#AD65BC" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" ForeColor="White" />
                <ItemStyle BackColor="#F4DFFF" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" HorizontalAlign="Center" VerticalAlign="Middle" 
                    Width="150px" />
                </asp:BoundField>
            <asp:BoundField HeaderText="สถานะการนำเข้า" DataField="IMPORT_STATUS" >
                <HeaderStyle BackColor="#AD65BC" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" ForeColor="White" />
                <ItemStyle BackColor="#F4DFFF" BorderColor="#666666" BorderStyle="Solid" 
                    BorderWidth="1px" HorizontalAlign="Center" VerticalAlign="Middle" 
                    Width="150px" />
                </asp:BoundField>
        </Columns>
            <HeaderStyle Height="30px" />
            <RowStyle Height="20px" VerticalAlign="Middle" />
    </asp:GridView>

using System;
using System.Data;
using CIM.Connection;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.DataVisualization.Charting;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Security;
using System.Configuration;
using System.Web.Services;
using System.Net;
using System.Web.Configuration;
using System.IO;
using System.Collections;
using System.Collections.Generic;
using System.Data.SqlClient;
using System.Text;
using SecurityEntities.Entities;
namespace BacklogMonitoring.Backlog
{
    public partial class Main : System.Web.UI.Page
    {
        string SQL;
        string SQL1;
        string SQL2;
        string newline = "\n";
        string getArea;
        string getAreaA;
        Connector dbConn;
        StdCtrl oStdCtrl;
        ImportData Import;
        XLS oXLS;
        string userLogin;
        DataTable dtArea = new DataTable();
        protected void Page_Load(object sender, EventArgs e)
        {
           

            {
                Connector dbConn = new OracleConnector();
                oStdCtrl = new StdCtrl();
                if (!IsPostBack)
                {
                    this.GetTopAreaA();
                    this.GetYears();
                    this.GetMyMonthList(MonthDD, true);
                    SQL2 = "select distinct(area) from PEA_ORGANIZE_CODE";
                    dtArea = dbConn.ExecuteQuery(SQL2);
                    getAreaA = dtArea.Rows[0][0].ToString();
                    this.TopAreaDD.SelectedValue = getAreaA;
                    LoadGridView();           //แสดงข้อมูลในรูปแบบของตาราง

                }
            }

        }



        .........................





        private void MergeGridviewRows(GridView gridView)
        {
            for (int rowIndex = gridView.Rows.Count - 2; rowIndex >= 0; rowIndex--)
            {
                GridViewRow row = gridView.Rows[rowIndex];
                GridViewRow previousRow = gridView.Rows[rowIndex + 1];

                if (row.Cells[0].Text == previousRow.Cells[0].Text && row.Cells[1].Text == previousRow.Cells[1].Text)
                {
                    row.Cells[0].RowSpan = previousRow.Cells[0].RowSpan < 2 ? 2 : previousRow.Cells[0].RowSpan + 1;
                    row.Cells[1].RowSpan = previousRow.Cells[1].RowSpan < 2 ? 2 : previousRow.Cells[1].RowSpan + 1;

                    previousRow.Cells[0].Visible = false;
                    previousRow.Cells[1].Visible = false;
                }

                if (row.Cells[0].Text == previousRow.Cells[0].Text)
                {
                    row.Cells[0].RowSpan = previousRow.Cells[0].RowSpan < 2 ? 2 : previousRow.Cells[0].RowSpan + 1;

                    previousRow.Cells[0].Visible = false;
                }
            }
        }

        protected void GridView3_PreRender(object sender, EventArgs e)
        {
            MergeGridviewRows(GridView3);
        }
       

        void Button3_Click(object sender, System.EventArgs e)
        {
            LoadGridView();
        }
 
    }
}