Erreur lors de l'éxecution

bonjour STP tu peut m'expliquer l'erreur, et d'avoir une solution. Merci d'avance.
L'erreur est le suivante : java.lang.NullPointerException
Le code est le suivante :
public class Gpop extends JFrame
{

class PopMessage {

public String message;
private String title;

public PopMessage (String titleW, String msg)
{
title = titleW;
message = msg;
}

public String toString()
{
return title;
}

}

public class PopServerDialog extends JDialog
{
JTextField hostTF, portTF, userTF;
JPasswordField passwordTF;

public PopServerDialog(Frame parent, boolean modal)
{
super(parent, modal);

JPanel panel1, panel2, panel3;
JButton okButton, cancelButton;

Container contentPane = this.getContentPane();

panel1 = new JPanel();
panel1.setLayout(new FlowLayout(FlowLayout.CENTER));
contentPane.add(panel1, BorderLayout.SOUTH);
okButton = new JButton("OK");
panel1.add(okButton);
cancelButton = new JButton("Cancel");
panel1.add(cancelButton);

panel3 = new JPanel();
panel3.setLayout(new FlowLayout(FlowLayout.CENTER));
contentPane.add(panel3, BorderLayout.CENTER);
hostTF = new JTextField(popServer, 20);
portTF = new JTextField("" + port, 5);
panel3.add(new JLabel("Server"));
panel3.add(hostTF);
panel3.add(new JLabel("Port"));
panel3.add(portTF);

panel2 = new JPanel();
panel2.setLayout(new FlowLayout(FlowLayout.CENTER));
contentPane.add(panel2, BorderLayout.NORTH);
userTF = new JTextField(user, 20);
passwordTF = new JPasswordField(password, 10);
panel2.add(new JLabel("User"));
panel2.add(userTF);
panel2.add(new JLabel("Passwd"));
panel2.add(passwordTF);

setTitle("Pop Server info");
pack();

cancelButton.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent event)
{
dispose();
setVisible(false);
}
});

okButton.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent event)
{
popServer = hostTF.getText();
user = userTF.getText();
password = new String(passwordTF.getPassword());

port = Integer.parseInt(portTF.getText());
dispose();
setVisible(false);
}
});

}
}

private String popServer = "pop.gmail.com";
private String user = "rimahddfdfd" , password = "*********";
private int port =995;

private JLabel status = new JLabel("Ready...");
private JTextArea Jbody = new JTextArea(10,20);
private Vector messages = new Vector();
private JList msgList = new JList();

public Gpop ()
{
JMenuBar menuBar = new JMenuBar();

JPanel statusbar = new JPanel();
JPanel Core = new JPanel();
JScrollPane SPbody, SPlist;

Core.setLayout(new BorderLayout());

Container contentPane = this.getContentPane();
contentPane.setLayout(new BorderLayout());

setJMenuBar(menuBar);

JMenu fMenu = new JMenu("File");
menuBar.add(fMenu);

JMenuItem popServerItem = new JMenuItem("Set Pop Server", 2);
fMenu.add(popServerItem);
JMenuItem popMsgItem = new JMenuItem("Get Messages", 2);
fMenu.add(popMsgItem);
fMenu.addSeparator();
JMenuItem exitItem = new JMenuItem("Exit", 2);
fMenu.add(exitItem);

statusbar.setLayout(new FlowLayout(FlowLayout.LEFT,5,5));
statusbar.setBorder(new BevelBorder(BevelBorder.LOWERED));
contentPane.add(statusbar,BorderLayout.SOUTH );
statusbar.add(status);

SPbody = new JScrollPane(Jbody,
JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
SPlist = new JScrollPane(msgList,
JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);

Core.add(SPlist, BorderLayout.WEST);
Core.add(SPbody, BorderLayout.EAST);

contentPane.add(Core);

// add listeners for the window and the various buttons
this.addWindowListener( new WindowAdapter(){
public void windowClosing(WindowEvent e){
setVisible(false);
dispose();
System.exit(0);
}});

exitItem.addActionListener( new ActionListener(){
public void actionPerformed(ActionEvent e){
System.exit(0);
}});

popMsgItem.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
getMessages();
}});

popServerItem.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
changePopServer();
}});

msgList.addMouseListener(new MouseAdapter() {
public void mouseClicked(MouseEvent e) {
int index = msgList.locationToIndex(e.getPoint());
if ( index >= 0 && index < messages.size())
Jbody.setText(((PopMessage)messages.elementAt(index)).message);
}
});

pack();

}

protected void getMessages()
{
PrintWriter to;
BufferedReader from;
String str,title, msg;
Vector v = new Vector();

try {
Socket socket = new Socket(InetAddress.getByName(popServer),port);

to = new PrintWriter(
new BufferedWriter (
new OutputStreamWriter (
socket.getOutputStream())),true);
from = new BufferedReader(
new InputStreamReader (
socket.getInputStream()));

while ( ! (from.readLine()).startsWith("+OK") );
to.println("USER "+user+"\r");
while ( ! (from.readLine()).startsWith("+OK") );
to.println("PASS "+password+"\r");
while ( ! (from.readLine()).startsWith("+OK") );

to.println("LIST\r");
while ( ! (from.readLine()).startsWith("+OK") );
do {
str = from.readLine();
if ( str.compareTo(".") != 0 ) v.add(str);
} while ( str.compareTo(".") != 0 );

for ( int i =0; i < v.size(); i++ ) {
title = (String ) v.elementAt(i);
to.println("RETR "+(new StringTokenizer(title)).nextToken()+"\r");
while ( ! (from.readLine()).startsWith("+OK") );
msg = "";
do {
msg += from.readLine() + "\n";
} while ( ! msg.endsWith("\n.\n") );
messages.add(new PopMessage(title, msg));
}

msgList.setListData(messages);

status.setText("Ready ...");

socket.close();

} catch ( Exception e ) {System.err.println(e);}

}

protected void changePopServer()
{
new PopServerDialog(this, true).show();
}

public static void main ( String args[] )
{
(new Gpop()).setVisible(true);

}

}