NullPointerException in my code. May I ask how to fix it?

Question

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Test implements ActionListener{
    static JFrame frm = new JFrame("Calculator");
    static JPanel jp1 = new JPanel(new FlowLayout());
    static JPanel jp2 = new JPanel(new GridLayout(3, 3));
    static JButton []btn = new JButton[9];
    static JTextField txtOut = new JTextField(15);

    static Container cp = frm.getContentPane();

    public static void main(String args[]){
        cp.setLayout(new BorderLayout(0, 10));

        jp1.add(txtOut);
        cp.add(jp1, BorderLayout.NORTH);

        for (int i = 0; i < 9; ++i){
            btn[i].setText("" + (i + 1));
            btn[i].addActionListener((ActionListener)frm);
            jp2.add(btn[i]);
        }
        cp.add(jp2, BorderLayout.CENTER);

        frm.setSize(200, 200);
        frm.setVisible(true);

        frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    public void actionPerformed(ActionEvent e){
        JButton tmp_btn = (JButton) e.getSource();
        // check which button
        for (int i = 0; i < 9; ++i){
            if (tmp_btn == btn[i]){
                txtOut.setText("" + (i + 1));
                break;
            }
        }
    }
}

Hello, I am studying in Java Swing course.

I tried to make a calculator,

but when I handle Event Listener, I face a problem!

It's java.lang.NullPointerException.

I guess the problem is in main method's for loop's three statements,

because I tried to comment them respectively,

and the IDE compiler talk me "NullPointerException" is in there...

But I still don't know what's wrong in this code.

So, I want to ask, ho to fix it, thanks!!

(Sorry I'm poor at English :(...