各位java大神请指点一下，急急急！谢谢！
我运行到下面一段代码的时候出现错误
Running with locale: Chinese (Simplified)_People's Republic of China.936
Running in the identified_third_party security domain
java.lang.NullPointerException
at com.han.Qunliao.commandAction(+39)
at javax.microedition.lcdui.Display$DisplayAccessor.commandAction(+282)
at javax.microedition.lcdui.Display$DisplayManagerImpl.commandAction(+10)
at com.sun.midp.lcdui.DefaultEventHandler.commandEvent(+68)
at com.sun.midp.lcdui.AutomatedEventHandler.commandEvent(+47)
at com.sun.midp.lcdui.DefaultEventHandler$QueuedEventHandler.handleVmEvent(+186)
at com.sun.midp.lcdui.DefaultEventHandler$QueuedEventHandler.run(+57)


代码如下


package com.han;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import javax.microedition.io.Connector;
import javax.microedition.io.SocketConnection;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Form;
import javax.microedition.lcdui.TextField;
import javax.microedition.midlet.MIDletStateChangeException;


public class Qunliao extends Form implements CommandListener,Runnable{
private static Qunliao instance;
private SocketConnection sc=null;
private DataOutputStream dos=null;
private DataInputStream dis=null;
private TextField txtName= new TextField("用  户  名：", "", 20, TextField.ANY);
private TextField txtMsg= new TextField("聊天信息：", "", 255, TextField.ANY); 
    private Command cmdSend = new Command("发送", Command.OK, 1);
private boolean isRUN=true;
public Qunliao() {

   // display=Display.getDisplay(this);
 super("群聊");
instance = this;
addCommand(cmdSend);
setCommandListener(this);
append(txtName);
append(txtMsg);
append("群聊区域:"+"\n");
MyConnection mc = new MyConnection();
    Thread t1= new Thread(mc);
   //  t1.start();

}
public void commandAction(Command c, Displayable d) {
// TODO Auto-generated method stub
try
{
dos.writeUTF(txtName.getString()+"说："+txtMsg.getString());
}catch(Exception ex)
{
ex.printStackTrace();
}

}
class MyConnection implements Runnable
{
    public MyConnection(){}
public void run() {
// TODO Auto-generated method stub
try
{
sc=(SocketConnection)Connector.open("socket://127.0.0.1:9999");
dis=sc.openDataInputStream();
dos=sc.openDataOutputStream();
new Thread(this).start();
}catch(Exception ex)
{
ex.printStackTrace();
}
}
}

public void run() {
// TODO Auto-generated method stub


while(isRUN)
{
try
{
String msg=dis.readUTF();
        append(msg+"\n");
}catch(Exception ex)
{
ex.printStackTrace();
}
}
}
public static Qunliao getInstance()