我不明白这个错误

Question

我正在尝试为学校的小程序分配获取背景图像，我收到一个我不明白的错误。如果相关，也使用Ready-to-Program

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

public class Summative extends Applet
    implements ActionListener
{
    Button bttn1 = new Button ("Formulas");
    Button bttn2 = new Button ("Start");
    boolean Formulas = false;
    boolean Start = false;
    URL url;
    Image background;
    public void init ()
    {
        setSize (1000, 1000);
        try
        {
            url = getDocumentBase ();
        }
        catch (Exception e)
        {
        }
        background = getImage(url, "MathMeme.JPEG");

        setBackground (Color.white);
        bttn1.addActionListener (this);
        bttn2.addActionListener (this);
        add (bttn1);
        add (bttn2);
    }


    public void paint (Graphics g)
    {
        g.setColor (Color.white);
        g.drawImage (background, 0, 0, this);
        /*if (Formulas == true)
        {
            Dimension d = getSize();
            g.setColor(Color.WHITE);
            g.fillRect(0, 0, d.width, d.height);
            g.drawString ("Enter Formulas here", 100, 100);
        }*/
    }


    public void actionPerformed (ActionEvent evt)
    {
        if (evt.getSource () == bttn1)
        {
            Formulas = true;
            repaint ();
        }
    }
}

这是我得到的错误：

java.lang.NullPointerException
    at sun.java2d.pipe.DrawImage.copyImage(Unknown Source)
    at sun.java2d.pipe.DrawImage.copyImage(Unknown Source)
    at sun.java2d.SunGraphics2D.drawImage(Unknown Source)
    at sun.java2d.SunGraphics2D.drawImage(Unknown Source)
    at Summative.paint(Summative.java:38)
    at sun.awt.RepaintArea.paint(Unknown Source)
    at sun.awt.windows.WComponentPeer.handleEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

我认为空指针异常是当你尝试并访问具有空值的东西，所以我不明白我如何在这个程序中有一个。