，j2me的Sprite精灵动画错误有关问题`

求助，j2me的Sprite精灵动画异常问题``
最近在学J2ME游戏基础，刚学到做一个简单的动画精灵就遇到问题了，很郁闷~！

请问下面代码哪里出问题了，我跟着视频教程一步步写的，为什么我异常，而人家却可以运行？？？？？
异常为：
Running with storage root C:\Documents and Settings\Administrator\j2mewtk\2.5.2\appdb\DefaultColorPhone
Running with locale: Chinese_People's Republic of China.936
Running in the identified_third_party security domain
java.lang.IllegalArgumentException
at javax.microedition.lcdui.game.Sprite.<init>(+41)
at spritepengzhuang.MIDlet4$MyGameCanvas.<init>(+60)
at spritepengzhuang.MIDlet4.<init>(+13)
at java.lang.Class.runCustomCode(+0)
at com.sun.midp.midlet.MIDletState.createMIDlet(+34)
at com.sun.midp.midlet.Scheduler.schedule(+52)
at com.sun.midp.main.Main.runLocalClass(+28)
at com.sun.midp.main.Main.main(+80)
java.lang.NullPointerException
at spritepengzhuang.MIDlet4$MyGameCanvas.<init>(+77)
at spritepengzhuang.MIDlet4.<init>(+13)
at java.lang.Class.runCustomCode(+0)
at com.sun.midp.midlet.MIDletState.createMIDlet(+34)
at com.sun.midp.midlet.Scheduler.schedule(+52)
at com.sun.midp.main.Main.runLocalClass(+28)
at com.sun.midp.main.Main.main(+80)

Java code

//动画的精灵同时也在运动，把一个连续的图片分成几个等分部分，import javax.microedition.lcdui.Display;import javax.microedition.lcdui.Graphics;import javax.microedition.lcdui.Image;import javax.microedition.lcdui.game.GameCanvas;import javax.microedition.lcdui.game.Sprite;import javax.microedition.midlet.MIDlet;import javax.microedition.midlet.MIDletStateChangeException;public class MIDlet4 extends MIDlet {    private MyGameCanvas mgc=new MyGameCanvas();    private Display dis;    public MIDlet4() {            }    protected void startApp() throws MIDletStateChangeException {        dis=Display.getDisplay(this);        dis.setCurrent(mgc);    }    protected void pauseApp() {            }    protected void destroyApp(boolean arg0) throws MIDletStateChangeException {            }    class MyGameCanvas extends GameCanvas implements Runnable{        private Image img;        private Sprite sp;        private Graphics gra;        private boolean RUN=true;        public MyGameCanvas(){            super(true);            gra=this.getGraphics();            try{                img=Image.createImage("/donghua.png");//放入要切割的图片                sp=new Sprite(img,img.getWidth()/4,img.getHeight());                        }catch(Exception ex){                ex.printStackTrace();                    }            new Thread(this).start();        }        public void run(){            while(RUN){                try{                                              //清屏                    gra.setColor(255,255,255);                    gra.fillRect(0, 0, this.getWidth(), this.getHeight());                    sp.paint(gra);                    this.flushGraphics();//缓冲区画                                        sp.move(1, 1);//移动                    sp.nextFrame();//动画效果                                        Thread.currentThread().sleep(100);                }catch(Exception ex){                    ex.printStackTrace();                        }            }        }    }}

Java code

img = Image.createImage("/donghua.png"); // 放入要切割的图片sp = new Sprite(img, img.getWidth() / 4, img.getHeight());