"ExecuteNonQuery: CommandText property has not been initialized"

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

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

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

                       


                        string strUpdate =
                            "Update CreateDocket set BranchCode = '" + strBranchCode + "', Date = '" + strDate + "', PKTS = '" + strPKTS + "',ActWt = '" + strActWt + "',ChargeWt = '" + strChargeWt + "',Mode = '" + strMode + "',ChallanNo = '" + strChallanNo + "',ChallanDate = '" + strChallanDate + "',VehicleNo = '" + strVehicleNo + "',Description = '" + strDescription + "' WHERE DEocketNo ='" + 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 txtActWT = (TextBox)grdRow.FindControl
                                              ("txtActWT");

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

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

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

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

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

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

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