Exception in thread "main" java.lang.NullPointerException

Question

import java.awt.*;
import java.awt.event.*;class Mypanel extends Panel implements ActionListener
{
    Button button1,button2;
    Color backColor;
    Mypanel()
    {
        button1=new Button("确定");
        button1=new Button("取消");
        add(button1);
        add(button2);
        button1.addActionListener(this);
        button2.addActionListener(this);
        
        setLayout(new FlowLayout());
        setBackground(Color.pink);
        backColor=getBackground();
    }
    public void actionPerformed(ActionEvent e)
    {
        if(e.getSource()==button1)
        {
            setBackground(Color.cyan);
        }  
        else if(e.getSource()==button2)
        {
            setBackground(backColor);
        }
            
    }    
}
class  WindowPanel extends Frame implements ActionListener
{
    Mypanel panel1,panel2;
    Button button;
    WindowPanel()
    {
        setLayout(new FlowLayout());
        panel1=new Mypanel();
        panel2=new Mypanel();
        button=new Button("退出");
        
        add(panel1);
        add(panel2);
        add(button);
        button.addActionListener(this);
        setBounds(60,60,200,200);
        setVisible(true);
        validate();
    }
    public void actionPerformed(ActionEvent e)
    {
        System.exit(0);
    }
}
public class Example7_10
{
    public static void main(String[] args)
    {
        new WindowPanel();
    }
}
运行是出现下面的错误：Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:1045)
at java.awt.Container.add(Container.java:365)
at Mypanel.<init>(Example7_10.java:13)
at WindowPanel.<init>(Example7_10.java:41)
at Example7_10.main(Example7_10.java:62)我也找不到哪个对象在使用前没初始化，请各位大虾帮忙

Answer 1

button2 = new Button("取消");

Answer 2

否则button2就是null；而button1则指向了另一个button“取消”