Java中的GridLayout

Question

package grid;


import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

public class Calc extends JFrame implements ActionListener {

/**
 * 
 */
private JTextField txtF1, txtF2, txtF3;
private JButton btn1, btn2, btn3, btn4, btn5, btn6;



public Calc() {
    // TODO Auto-generated constructor stub
    super("Grid Layout Calculator");
    Container c = getContentPane();
    c.setLayout(new GridLayout(3, 3));
    setContentPane(c);
    c.setLayout(null);

    JPanel panel1 = new JPanel(), panel2 = new JPanel(), panel3 = new JPanel(), panel4 = new JPanel(), panel5 = new JPanel(), panel6 = new JPanel(), panel7 = new JPanel(), panel8 = new JPanel(), panel9 = new JPanel();


    btn1 = new JButton("Add");
    btn1.addActionListener(this);
    btn2 = new JButton("Subtract");
    btn2.addActionListener(this);
    btn3 = new JButton("Multiply");
    btn3.addActionListener(this);
    btn4 = new JButton("Divide");
    btn4.addActionListener(this);
    btn5 = new JButton("Modulus");
    btn5.addActionListener(this);
    btn6 = new JButton("=");
    btn6.addActionListener(this);

    panel1.setLayout(new GridLayout(1, 1));
    panel1.add(txtF1);
    panel2.setLayout(new GridLayout(1, 2));
    panel2.add(txtF2);
    panel3.setLayout(new GridLayout(1, 3));
    panel3.add(txtF3);
    panel4.setLayout(new GridLayout(2, 1));
    panel4.add(btn1);
    panel5.setLayout(new GridLayout(2, 2));
    panel5.add(btn2);
    panel6.setLayout(new GridLayout(2, 3));
    panel6.add(btn3);
    panel7.setLayout(new GridLayout(3, 1));
    panel7.add(btn4);
    panel8.setLayout(new GridLayout(3, 2));
    panel8.add(btn5);
    panel9.setLayout(new GridLayout(3, 3));
    panel9.add(btn6);

    c.add(panel1);
    c.add(panel2);
    c.add(panel3);
    c.add(panel4);
    c.add(panel5);
    c.add(panel6);
    c.add(panel7);
    c.add(panel8);
    c.add(panel9);

    setSize(500,250);
    show();
    }

    public void actionPerformed(ActionEvent e) {
    if (e.getSource() == btn1){
        int x = Integer.parseInt(txtF1.getText());
        int y = Integer.parseInt(txtF2.getText());

        int sum = x + y;
        txtF3.setText(String.valueOf(sum));
    }
    else if (e.getSource() == btn2){
        int x = Integer.parseInt(txtF1.getText());
        int y = Integer.parseInt(txtF2.getText());

        int difference = x - y;
        txtF3.setText(String.valueOf(difference));
    }
    else if (e.getSource() == btn3){
        int x = Integer.parseInt(txtF1.getText());
        int y = Integer.parseInt(txtF2.getText());

        int product = x * y;
        txtF3.setText(String.valueOf(product));
    }
    else if (e.getSource() == btn4){
        int x = Integer.parseInt(txtF1.getText());
        int y = Integer.parseInt(txtF2.getText());

        int quotient = x / y;
        txtF3.setText(String.valueOf(quotient));
    }
    else if (e.getSource() == btn5){
        int x = Integer.parseInt(txtF1.getText());
        int y = Integer.parseInt(txtF2.getText());

        int mod = x % y;
        txtF3.setText(String.valueOf(mod));
    }

    }

    public static void main(String[] args) {
    Calc app = new Calc();
    app.addWindowListener(
            new WindowAdapter() {
                public void windowClosing(WindowEvent e)
                {
                    System.exit(0);
                }
            });
       }

       }

我收到这些错误

Exception in thread "main" java.lang.NullPointerException
    at java.awt.Container.addImpl(Unknown Source)
    at java.awt.Container.add(Unknown Source)
    at grid.Calc.<init>(Calc.java:49)
    at grid.Calc.main(Calc.java:121)