null pointer exception

[kelvin]

hello, i need some help...

now i made j2me application with bluetooth, and i have a problem, when i want input data in handphone application i get null pointer exception statement, this program can be execute and no error, i not understand what this problem....

please help me, i give this code is follow :

import javax.microedition.midlet.MIDlet;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.*;
import java.io.*;

public final class LibraryClient extends MIDlet implements CommandListener {
static final int ALERT_TIMEOUT = 2000;
private final Command KELUAR_CMD = new Command("Keluar",Command.EXIT,1 );
private final Command OK_CMD = new Command( "Masuk", Command.SCREEN, 2 );
private final Form menu = new Form( "Bluetooth Library" );
private LibraryGUI LibraryBTClient = null;
private Alert a;

public LibraryClient() {
menu.addCommand( KELUAR_CMD );
menu.addCommand( OK_CMD );
menu.setCommandListener( this );
a = null;
}

public void startApp() {
show( a );}

public void pauseApp() {}

public void destroyApp( boolean unconditional ) {
if ( LibraryBTClient != null ) {
LibraryBTClient.destroy();}}

public void commandAction( Command c, Displayable d ) {
if ( c == KELUAR_CMD ){
destroyApp( true );
notifyDestroyed();}
if ( c == OK_CMD ){
LibraryBTClient = new LibraryGUI( this );}}

void show( Alert a ) {
if ( a == null ){
Display.getDisplay(this).setCurrent(menu);}
else{
Display.getDisplay(this).setCurrent( a, menu );}}
Displayable getDisplayable(){
return menu;}
}


thanks...