there is an error object reference not set to an instance of object

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;

namespace WindowsFormsApplication1
{
    public partial class Form5 : Form
    {
        string connetionString = null;
        SqlConnection cn;
        SqlCommandBuilder cb;
        DataSet s1;
        SqlDataAdapter da;


        public Form5()
        {
            InitializeComponent();
        }
        private void form5_Load(object Sender, EventArgs e)
        {
            string connectionString = null;
            SqlConnection cn;
            connectionString = "Data Source=SHAKIR-PC\\SQLEXPRESS;Initial Catalog=UDdata;Integrated Security=True;Pooling=False";
            cn = new SqlConnection(connectionString);
            string sqlQuerry = "select * from UDdata";
            da = new SqlDataAdapter(sqlQuerry, cn);

            s1 = new DataSet();
            da.Fill(s1, "UDdata");
        }

        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                cb = new SqlCommandBuilder(da);
                // to insetr the details in form to the datavase tables we need an object
                DataRow drow = s1.Tables["customer"].NewRow();
                drow[0] = textBox1.Text;
                drow[1] = textBox2.Text;
                drow[2] = textBox3.Text;

                s1.Tables["customer"].Rows.Add(drow);
                da.Update(s1, "customer");
                MessageBox.Show("Data Successfully added", "Done");

            }
            catch (Exception ee)
            {
                MessageBox.Show("There is an error " + ee.Message);
            }
            textBox1.Text = "";
            textBox2.Text = "";
            textBox3.Text = "";
        }
    }
}