null pointer exception in thread

import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;
import java.util.*;

public class AppServer extends Frame implements ActionListener,
Runnable{

    /**
     * @param args
     */

    Button b1;
    TextField tf;
    TextArea ta;
    ServerSocket ss;
    Socket s;
    PrintWriter pw;
    Scanner input;
    Thread th;

    public AppServer() {
        Frame f = new Frame("Server side chatting");
        f.setLayout(new FlowLayout());
        f.setBackground(Color.ORANGE);
        b1 = new Button("Send");
        b1.setBackground(Color.PINK);
        b1.addActionListener(this);
        tf = new TextField(30);
        ta = new TextArea();
        ta.setBackground(Color.CYAN);
        f.addWindowListener(new W1());
        f.add(tf);
        f.add(b1);
        f.add(ta);
        try {
            ss = new ServerSocket(12000);
            s = ss.accept();
            input = new Scanner(s.getInputStream());
            pw = new PrintWriter(s.getOutputStream(),true);
        }catch(Exception e) {
            e.printStackTrace();
        }

        setFont(new Font("Arial", Font.BOLD,20));
        f.setSize(200,200);
        f.setLocation(300,300);
        f.setVisible(true);
        f.validate();
        th = new Thread(this);
        th.setDaemon(true);
        th.start();
    }

    private class W1 extends WindowAdapter {
        public void windowClosing(WindowEvent we) {
            System.exit(0);
        }
    }

    public void actionPerformed(ActionEvent ae) {
        pw.println(tf.getText());
        tf.setText("");
    }

    public void run() {
        while(true) {
            try {
                ta.append(input.nextLine());
            }catch(Exception e) {
                e.printStackTrace();
            }
        }
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        AppServer a = new AppServer();

    }

}

In the above code, I have used a thread which updates the text area if
some data is received. Why it shows the following exception ???

java.lang.NullPointerException
    at AppServer.run(AppServer.java:70)
    at java.lang.Thread.run(Thread.java:619)