NullPointerException

[qp]

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

public class Facharbeit extends Applet implements ActionListener
{
    GUIPanel p;
    Graphics g;
    int[][] wertetabelle;
    int yPunkt;

public void init()
    {
        setLayout(null);
        setSize(600,440);
        p=new GUIPanel();
        add(p);
        p.setzeActionListener(this);
        Graphics g = p.getGraphics();
        wertetabelle = new int[6][2];
}

...
public void zeichneY(Graphics g)
     {
         int vgl=50+yPunkt();
    g.fillRect(326,50,3,300);
}
...
public void test()
{
init();
zeichneY(g);
}
...
public void actionPerformed(ActionEvent e)
 {
    Button b = (Button)e.getSource();
    if (b==p.b1)
    {
        g.fillRect(0,0,250,250);
        test();

}
}
...
class GUIPanel extends Panel
{
Button b1,b2;
Graphics g;

public GUIPanel()
    {
...}