NullPointerException.... in java

[ketand1]

import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.util.*;
import java.sql.*;
import java.lang.*;
 
class DbAwt extends Frame implements ActionListener
{
    private TextField t1, t2, t3;
    private Button b1, b2, b3;
    private Label l1, l2, l3;
    private Connection con = null;
    private ResultSet rs;
    private Statement st;
    private String name = null;
    private PreparedStatement pst = null;
    private int r = 0, cr = 0;
 
    public DbAwt()
    {
 
        t1 = new TextField();
        t2 = new TextField();
        t3 = new TextField();
        b1 = new Button("Insert");
        //b2 = new Button("Update");
        b3 = new Button("Delete");
        l1 = new Label("Roll Number");
        l2 = new Label("Name");
        l3 = new Label("Cet Score");
 
        setBackground(Color.gray);
        setSize(400, 400);
 
        setLayout(new GridLayout(6, 6, 50, 40));
        add(l1);
        add(t1);
        add(l2);
        add(t2);
        add(l3);
        add(t3);
        add(b1);
        add(b3);
        add(b2);
        b1.addActionListener(this);
        b3.addActionListener(this);
        addWindowListener(new winevent());
        setVisible(true);
    }
    public void actionPerformed(ActionEvent ae)
    {
        try
        {
            if (ae.getSource() == b1)
            {
                Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
                con = DriverManager.getConnection("Jdbc:Odbc:ketan");
                r = Integer.parseInt(t1.getText());
                name = t2.getText();
                cr = Integer.parseInt(t3.getText());
                pst = con.prepareStatement("insert into Stud (Roll,Name,cetscr) values(?,?,?)");
                pst.setInt(1, r);
                pst.setString(2, name);
                pst.setInt(3, cr);
                int n = 0;
                n=pst.executeUpdate();
 
                if (n > 0)
                {
                    //t1.setText("");
                    //t2.setText("");
                    //t3.setText("");
                    t1.setText("Data inserted...");
                }
                else
                {
                    //t1.setText("");
                    //t2.setText("");
                    //t3.setText("");
                    t1.setText("Data inserted...");
                }
                con.close();
            }
            else
            {
                t1.setText("");
                t2.setText("Good bye");
            }
        }
        catch (Exception e)
        {
            System.out.println("Exception is : " + e);
        }
    }
    public static void main(String arg[])throws NullPointerException
    {
        try
        {
            new DbAwt();
        }
        catch (NullPointerException ne)
        {
            System.out.println("Exception is : " + ne);
        }
    }
}
class winevent extends WindowAdapter
{
    public void windowClosing(WindowEvent we)
    {
        System.exit(0);
    }
}

When i run this programm at run time i get the following error please explain me hou to overcome from this error

 Exception in thread "main" java.lang.NullPointerException
        at java.awt.Container.addImpl(Container.java:1027)
        at java.awt.Container.add(Container.java:352)
        at DbAwt1.<init>(DbAwt1.java:43)
        at DbAwt1.main(DbAwt1.java:49)

[r035198x]

1.) Use code tags when posting code
2.) Use meaningful variable names in your code. Names like b1,b2 e.t.c create confusion for everyone including yourself.
3.) Nullpointers are a sitting duck if you know how to use System.out.println to print out variable values.
4.) Whay did you not initialize b2 if you wanted to add it to your container?