j2me sprite not working - java-me

Question

First, I'm sorry for my bad English. I'm a newbie J2ME
I wrote the below code after i watched some tutorial but it not working. I don't know what happen with it.
AnimationCanvas.java
package example;
import java.io.IOException;
import javax.microedition.lcdui.*;
import javax.microedition.lcdui.game.GameCanvas;
import javax.microedition.lcdui.game.LayerManager;
import javax.microedition.lcdui.game.Sprite;
public class AnimationCanvas extends GameCanvas implements Runnable {
private Sprite ca;
private int frameWidth = 14;
private int frameHeight = 14;
private boolean playing = true;
private int a=1;
public AnimationCanvas(){
super(true);
try {
ca = new Sprite(Image.createImage("ca.png"), frameWidth, frameHeight);
} catch (IOException ex) {
ex.printStackTrace();
}
}
public void start() {
Thread runner = new Thread(this);
runner.start();
}
public void run(){
while (playing){
drawDisplay(getGraphics());
try{
Thread.sleep(10);
} catch(Exception e){}
}
}
public void drawDisplay(Graphics g){
g.setColor(0x000000);
g.fillRect(0,0,getWidth(),getHeight());
ca.setFrame(1);
ca.paint(g);
flushGraphics();
}
public void stop(){
playing=false;
}
}
and AnimationMidlet.java
package example;
import java.io.IOException;
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
public class AnimationMidlet extends MIDlet {
private Display display;
private AnimationCanvas canvas;
public AnimationMidlet() throws IOException {
display = Display.getDisplay(this);
canvas = new AnimationCanvas();
}
private void initialize() {
// write pre-initialize user code here
// write post-initialize user code here
}
public void startMIDlet() {
// write pre-action user code here
// write post-action user code here
}
public void resumeMIDlet() {
// write pre-action user code here
// write post-action user code here
}
public void switchDisplayable(Alert alert, Displayable nextDisplayable) {
// write pre-switch user code here
Display display = getDisplay();
if (alert == null) {
display.setCurrent(nextDisplayable);
} else {
display.setCurrent(alert, nextDisplayable);
}
// write post-switch user code here
}
public Display getDisplay() {
return Display.getDisplay(this);
}
public void startApp() {
if(canvas!=null){
display.setCurrent(canvas);
canvas.start();
}
}
public void pauseApp() {
}
public void destroyApp(boolean unconditional) {
canvas.stop();
}
}
and the output
Running in the identified_third_party security domain
java.io.IOException
Uncaught exception java/lang/NullPointerException.
at javax.microedition.lcdui.ImmutableImage.getImageFromStream(Image.java:968)
at javax.microedition.lcdui.ImmutableImage.(Image.java:939)
at javax.microedition.lcdui.Image.createImage(Image.java:313)
at example.AnimationCanvas.(AnimationCanvas.java:25)
at example.AnimationMidlet.(AnimationMidlet.java:25)
at java.lang.Class.runCustomCode(+0)
at com.sun.midp.midlet.MIDletState.createMIDlet(+34)
Thanks for your reading!