调用JPanel.getGraphics()出现java.lang.NullPointerException

各位大大，我想画个网格，但是
Graphics graphics = panel.getGraphics();
这条语句却报出java.lang.NullPointerException？
不知什么原因，求解，谢谢.
package test;import java.awt.*;
import javax.swing.*;public class TestGrid extends JFrame{ public static void main(String[] args) {  
        TestGrid game = new TestGrid();  
        game.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  
        game.setVisible(true);  
    } 

public TestGrid(){
super();
this.setTitle("just a test!");
this.setSize(600, 400);
this.getContentPane().setLayout(new BorderLayout()); DrawGrid(resultPanel);
this.add(resultPanel,BorderLayout.CENTER);
}
private void DrawGrid(JPanel panel) {
width = panel.getWidth();
height = panel.getHeight(); section_width = width / 20;
section_height = height / 20; remainder_width = width - section_width * 20;
remainder_height = height - section_height * 20; Graphics graphics = panel.getGraphics();
graphics.clearRect(0, 0, width, height);

for (int i = 0; i <= 20; i++) {
graphics.drawLine(remainder_width / 2 + section_width * i,
remainder_height / 2, 
remainder_width / 2 + section_width * i,
height - remainder_height / 2);
} for (int i = 0; i <= 20; i++) {
graphics.drawLine(remainder_width / 2, 
remainder_height / 2 + section_height * i,
width - remainder_width / 2,
remainder_height / 2 + section_height * i);
}
}

private int width ;
private int height ;
private int section_width;
private int section_height;
private int remainder_width;
private int remainder_height;
private JPanel resultPanel = new JPanel();
}