Problems using Command Listener. :(

[sourcecode=javax]
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class MainBase extends MIDlet implements Runnable, CommandListener
{
	private Display desplegar;
	private Command comandoSalir;//Comando de Salida
	
	private GameAlfaClass program;
	public int opcion=99;
	public boolean isPlay=true;
	public boolean isOut=true;
	public void startApp() throws MIDletStateChangeException
	{
		desplegar = Display.getDisplay(this);
		program=new GameAlfaClass(this);
		comandoSalir = new Command("Salir",Command.EXIT,1);
		program.addCommand(comandoSalir);
		program.setCommandListener(this);
		Thread t = new Thread(this);
	    t.start();
	}
	public void run()
	{
		try
		{
			program.start();
			desplegar.setCurrent(program);    
			isOut = true;
			while (isOut){}    
			program.stop();
			program = null;

		}
		catch (Exception ex)
		{
			System.out.println(ex);
			notifyDestroyed();
		}

		
	}
	public Display getDisplay()
	{
		return desplegar;  
	}
	public void pauseApp()
	{
	}
	public void destroyApp(boolean unconditional)
	{
		notifyDestroyed();
	}
	public void commandAction(Command c, Displayable s) 
	{
		if (c == comandoSalir) 
		{
			destroyApp(true);
			notifyDestroyed();
		}
	}
}


[/sourcecode]

The GAMEALFACLASS works like this:

public class GameAlfaClass extends GameCanvas implements Runnable


======================

The problem comes when i run this app on my Nokia 6670. When i use the command "comandoSalir", i get this two errors:
1) Buffer closed java.lang.RunTimeException !

2) 0 java.lang.NullPointerException !


Anyone knows something about this?

Thanks