After saving data in gridview making changes in gridview

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ASP.NET

C#5.0

SQL-Server-2008R2

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;
using System.Text;

public partial class CreateChallan : System.Web.UI.Page
{
    string connStr = ConfigurationManager.ConnectionStrings["amitpandeyConnectionString2"].ConnectionString;
      protected void btnUpdate_Click(object sender, EventArgs e)
        {
            StringBuilder strSql = new StringBuilder(string.Empty);

            SqlConnection con = new SqlConnection(connStr);
            SqlCommand cmd = new SqlCommand();

            for (int i = 0; i < GridView1.Rows.Count; i++)
            {
                CheckBox chkUpdate = (CheckBox)
                   GridView1.Rows[i].Cells[0].FindControl("chkSelect");
                if (chkUpdate != null)
                {
                    if (chkUpdate.Checked)
                    {

                        string strDocketNo = GridView1.Rows[i].Cells[1].Text;

                        string strBranchCode = ((TextBox)GridView1.Rows[i].FindControl("txtBranchCode")).Text;

                        string strDate = ((TextBox)GridView1.Rows[i].FindControl("txtDate")).Text;

                        string strPKTS = ((TextBox)GridView1.Rows[i].FindControl("txtPKTS")).Text;

                        string strTo = ((TextBox)GridView1.Rows[i].FindControl("txtTo")).Text;

                        string strActWt = ((TextBox)GridView1.Rows[i].FindControl("txtActwt")).Text;

                        string strChargeWt = ((TextBox)GridView1.Rows[i].FindControl("txtChargeWt")).Text;

                        string strMode = ((TextBox)GridView1.Rows[i].FindControl("Mode")).Text;

                        string strChallanNo = ((TextBox)GridView1.Rows[i].FindControl("ChallanNo")).Text;

                        string strVehicleNo = ((TextBox)GridView1.Rows[i].FindControl("VehicleNo")).Text;

                        string strChallanDate = ((TextBox)GridView1.Rows[i].FindControl("ChallanDate")).Text;

                        string strDescription = ((TextBox)GridView1.Rows[i].FindControl("Description")).Text;                      

                        string strUpdate ="Update CreateDocket set BranchCode = '" + strBranchCode + "', Date = '" + strDate + "', PKTS = '" + strPKTS + "',To = '"+strTo+"',ActWt = '" + strActWt + "',ChargeWt = '" + strChargeWt + "',Mode = '" + strMode + "',ChallanNo = '" + strChallanNo + "',VehicleNo = '" + strVehicleNo + "',ChallanDate = '" + strChallanDate + "',Description = '" + strDescription + "' WHERE DocketNo ='" + strDocketNo + "'";
                        strSql.Append(strUpdate);
                    }
                }
            }
            try
            {
                cmd.CommandType = CommandType.Text;
                cmd.CommandText = strSql.ToString();
                cmd.Connection = con;
                con.Open();
                cmd.ExecuteNonQuery();
            }
            catch (SqlException ex)
            {
                string errorMsg = "Error in Updation";
                errorMsg += ex.Message;
                throw new Exception(errorMsg);
            }
            finally
            {
                con.Close();
            }
            
        }
       
        protected void chkSelect_CheckedChanged
                            (object sender, EventArgs e)
        {
            CheckBox chkTest = (CheckBox)sender;
            GridViewRow grdRow = (GridViewRow)chkTest.NamingContainer;

            TextBox txtBranchCode = (TextBox)grdRow.FindControl("txtBranchCode");

            TextBox txtDate = (TextBox)grdRow.FindControl("txtDate");

            TextBox txtPKTS = (TextBox)grdRow.FindControl("txtPKTS");

            TextBox txtTo = (TextBox)grdRow.FindControl("txtTo");

            TextBox txtActWT = (TextBox)grdRow.FindControl("txtActWT");

            TextBox txtChargeWt = (TextBox)grdRow.FindControl("txtChargeWt");

            TextBox txtMode = (TextBox)grdRow.FindControl("txtMode");

            TextBox txtChallanNo = (TextBox)grdRow.FindControl("txtChallanNo");

            TextBox txtVehicleNo = (TextBox)grdRow.FindControl("txtVehicleNo");

            TextBox txtChallanDate = (TextBox)grdRow.FindControl("txtChallanDate");            

            TextBox txtDescription = (TextBox)grdRow.FindControl("txtDescription");

            if (chkTest.Checked)
            {
              
                txtChallanNo.ReadOnly = false;
                txtVehicleNo.ReadOnly = false;
                txtChallanDate.ReadOnly = false;                
                txtDescription.ReadOnly = false;
                txtChallanNo.ForeColor = System.Drawing.Color.Red;
                txtVehicleNo.ForeColor = System.Drawing.Color.Red;
                txtChallanDate.ForeColor = System.Drawing.Color.Red;                
                txtDescription.ForeColor = System.Drawing.Color.Red;
            }
            else
            {
                txtChallanNo.ReadOnly = true;
                txtVehicleNo.ReadOnly = true;
                txtChallanDate.ReadOnly = true;                
                txtDescription.ReadOnly = true;
                txtChallanNo.ForeColor = System.Drawing.Color.Green;
                txtVehicleNo.ForeColor = System.Drawing.Color.Green;
                txtChallanDate.ForeColor = System.Drawing.Color.Green;                
                txtDescription.ForeColor = System.Drawing.Color.Green;  
                
            }
        }
    }

What I have Done is I Have made some entries in last page and I have left some entries for next page But my problem is Now when I write in text boxes then its showing me two error 1st is "Object reference not set to an instance of an object". for VehicleNO
2nd is It is showing me null reference For ChallanNO
Please Help me .