How to create a lookup in ASP.NET using C#

object is not set to an instance of an object

public static void fill(string property)
        {
            Page page = (Page)HttpContext.Current.Handler;
            TextBox TenancyAddress = (TextBox)page.FindControl("TenancyAddress");
            TenancyAddress.Text = property;
        }

        [WebMethod]
        public static void LookupUpdate(string id)
        {
            SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["con"].ToString());
            con.Open();
            SqlCommand cmd = new SqlCommand("select * from [godaddy].[dbo].[Property] where [Prop_id] = " + id, con);
            SqlDataAdapter sda = new SqlDataAdapter();
            cmd.Connection = con;
            sda.SelectCommand = cmd;
            DataSet ds = new DataSet();
            sda.Fill(ds);

            if (ds.Tables[0].Rows.Count > 0)
            {
                Page page = (Page)HttpContext.Current.Handler;
                TextBox TenancyAddress = (TextBox)page.FindControl("TenancyAddress");
                for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
                {
                 string property = ds.Tables[0].Rows[i]["PropertyName"].ToString();
                    fill(property);
                }
            }
            con.Close();
            



        }