疑惑。这为啥有空指针异常？哪里出错了呢

//银行贷款按月还本复习的计算import java.util.*;
import java.awt.Color;
import java.awt.event.*;
import javax.swing.*;public class LoanJFrame extends JFrame implements ActionListener{ /**
 * 
 */
private static final long serialVersionUID = 1L;
/**
 * @param args
 */
private JTextField text_money,text_rate,text_year;           //贷款金额，利率，年限文本行；
private JSpinner spin_year,spin_month;
private JButton button;
private JTable table;

public LoanJFrame()
{
    super("银行贷款按月还本付息的计算");
    this.setBounds(300,240,740,400);
    this.setBackground(Color.LIGHT_GRAY);
    this.setDefaultCloseOperation(EXIT_ON_CLOSE);
    JPanel panel=new JPanel();
    this.getContentPane().add(panel,"North");
    panel.add(new JLabel("贷款金额"));
    text_money=new JTextField("100000",6);
    panel.add(text_money);
    
    panel.add(new JLabel("元    贷款利率"));
    text_rate=new JTextField("0.05025",6);
    panel.add(text_rate);
    panel.add(new JLabel("%/月   贷款年限"));
    text_year=new JTextField("5",3);
    panel.add(text_year);
    panel.add(new JLabel("年   起始年月"));
    
    Calendar now =Calendar.getInstance();           //获得当前日期
    int year=now.get(Calendar.YEAR);
    int month=now.get(Calendar.MONTH)+1;
    int nextmonth=month%12+1;                       //下月
    if(nextmonth==1)
     year++;
    
    spin_year=new JSpinner();
    spin_year.setValue(year);
    panel.add(spin_year);
    panel.add(new JLabel("年"));
    spin_month=new JSpinner(new SpinnerNumberModel(nextmonth,1,12,1));      //数值模式初值为下月，范围是1-12，变化值为1.
    panel.add(spin_month);
    panel.add(new JLabel("月"));
    button=new JButton("计算");
    panel.add(button);
    button.addActionListener(this);
    
    actionPerformed(null);     this.getContentPane().add(new JScrollPane(table));
    this.setVisible(true);
    
    
}

public void actionPerformed(ActionEvent e)
{
String titles[]={"年月","本金余额（元）","月还本金（元）","月还利息（元）","月还本息（元）"};
int months=Integer.parseInt(text_year.getText())*12;     //还款月数
Object datas[][]=new Object[months][];                   //创建存储表格数据项的二维数组
int year=Integer.parseInt(""+spin_year.getValue());         //起始年份
int mon=Integer.parseInt(""+spin_month.getValue());          //起始月份
double leaving=Double.parseDouble(""+text_money.getText());     //本金余额
double pay=leaving/months;                                   //月还本金
double rate =Double.parseDouble(""+text_rate.getText());     //贷款利率
for(int i=0;i<months;i++)
{
datas[i][0]=year+"年"+mon+"月";   datas[i][1]=String.format("%9.2f", leaving);            
datas[i][2]=String.format("%9.2f", pay);                         //月还本金
datas[i][3]=String.format("%9.2f", leaving*rate*0.01);        //月还利息
datas[i][4]=String.format("%9.2f", pay+leaving*rate*0.01);         //月还本息
if(mon==12)
year++;
mon=mon%12+1;
leaving-=pay;

}
table=new JTable(datas,titles);
}
public static void main(String[] args) {
// TODO Auto-generated method stub
   new LoanJFrame();
}}
Exception in thread "main" java.lang.NullPointerException
at LoanJFrame.actionPerformed(LoanJFrame.java:79)
at LoanJFrame.<init>(LoanJFrame.java:60)
at LoanJFrame.main(LoanJFrame.java:94)指针异常