ผิดพลาด Object reference not set to an instance of an object. แก้อย่างไงหรอค่ะ

public void updateList(object sender, EventArgs e)
        {
DropDownList ddl = (DropDownList)Repeater2.FindControl("DropDownList1");
            TextBox up_name = (TextBox)Repeater2.FindControl("th_name");
            TextBox up_date_st = (TextBox)Repeater2.FindControl("date_st");
            TextBox up_date_ed = (TextBox)Repeater2.FindControl("date_ed");
            string stat = ddl.SelectedValue;
            string tc = up_name.ToString();
            string date_st = up_date_st.ToString();
            string date_ed = up_date_ed.ToString();

            string connection = WebConfigurationManager.ConnectionStrings["Project"].ConnectionString;
            SqlConnection conn = new SqlConnection(connection);
            if (conn.State == ConnectionState.Open)
            {
                conn.Close();
            }
            conn.Open();
            string cmdText = "Update Order_P SET Order_status = " + stat;
            cmd.ExecuteNonQuery();
            string cmdText = "Update OrderList SET tc_name = " + tc
                      + "OrderList_date_start = " + date_st
                      + "OrderList_date_end = " + date_ed;
            cmd.ExecuteNonQuery();
            conn.Close();
        }