关于Server与Client的异常

import javax.swing.*;//客户端程序
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;

public class Client{
boolean a_key=false;
Client(){
Socket socket=null;
InputStreamReader ir=null;
BufferedReader br=null;
OutputStreamWriter ow=null;
BufferedWriter bw=null;
final PrintWriter pw=new PrintWriter(bw,true);

JFrame f1=new JFrame("客户端");
Container p1=f1.getContentPane();
final JTextArea t1=new JTextArea(30,40);
JButton b1=new JButton("发送");
JPanel panel=new JPanel();
panel.add(b1);
p1.add(panel,BorderLayout.SOUTH);
p1.add(t1);
f1.setSize(612,450);
f1.setVisible(true);
f1.pack();
System.out.println("KKK");
f1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
try{

InetAddress addr=InetAddress.getByName(null);
socket=new Socket(addr,8888);
//创建个套接字，连接到本机的8888端口
InputStream in=socket.getInputStream();
ir=new InputStreamReader(in);
br=new BufferedReader(ir);
OutputStream out=socket.getOutputStream();
ow=new OutputStreamWriter(out);
bw=new BufferedWriter(ow);
String line=null;
StringBuilder sb=new StringBuilder();
if(sb!=null){
while((line=br.readLine())!=null){
if(line.equals("要签到了")){

JFrame f2=new JFrame("签到");
Container p2=f2.getContentPane();
JLabel label2=new JLabel("快点我吧!");
JButton b2=new JButton("签到");
p2.add(label2);
p2.add(b2);
f2.setSize(300,220);
f2.setVisible(true);
f2.pack();
f2.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

b2.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){boolean a_key=false;
a_key=true;
}
});
if(a_key)
pw.println("签到了哈哈呵呵");
else
pw.println("签到失败呜呜呜");
}
sb.append(line+"\n");
}
}
String sa=null;
sa=sb.toString();
t1.append(sa);
}catch(Exception ce){ce.printStackTrace();}
b1.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
String sa=null;
sa=t1.getText();
pw.println("服务端："+sa);
}
});
}
public static void main(String[] args){
new Client();
}
}
报错： C:\java>java Client
Exception in thread "main" java.lang.NullPointerException
at java.io.Writer.<init>(Writer.java:71)
at java.io.PrintWriter.<init>(PrintWriter.java:75)
at Client.<init>(Client.java:15)
at Client.main(Client.java:84)



求指点，本人新手，java才开始学