Java socket - simple program doesn't work

I've spent lot of time to find out where is the problem but with no success. Server is launching correctly, but when I launch Client I get "Unexpected Error" exception. I've changed ports too with no effects. What should I do to make this working?

/* Server.java */
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class Server
{
private static final int PORT = 50000;
static boolean flaga = true;

private static ServerSocket serverSocket;
private static Socket clientSocket;

public static void main(String[] args) throws IOException
{
    serverSocket = null;
    try
    {
        serverSocket = new ServerSocket(PORT);
    }
    catch(IOException e)
    {
        System.err.println("Could not listen on port: "+PORT);
        System.exit(1);
    }

    System.out.print("Wating for connection...");

    Thread t = new Thread(new Runnable()
    {
        public void run()
        {
            try
            {
                while(flaga)
                {
                    System.out.print(".");
                    Thread.sleep(1000);
                }
            }
            catch(InterruptedException ie)
            {
                //
            }

            System.out.println("\nClient connected on port "+PORT);
        }
    });
    t.start();

    clientSocket = null;
    try
    {
        clientSocket = serverSocket.accept();
        flaga = false;
    }
    catch(IOException e)
    {
        System.err.println("Accept failed.");
        t.interrupt();
        System.exit(1);
    }

    final PrintWriter out = new PrintWriter(clientSocket.getOutputStream(),true);
    final BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

    t = new Thread(new Runnable()
    {
        public void run()
        {
            try
            {
                Thread.sleep(5000);

                while(true)
                {
                    out.println("Ping");
                    System.out.println(System.currentTimeMillis()+" Ping sent");

                    String input = in.readLine();

                    if(input.equals("Pong"))
                    {
                        System.out.println(System.currentTimeMillis()+" Pong received");
                    }
                    else
                    {
                        System.out.println(System.currentTimeMillis()+" Wrong answer");

                        out.close();
                        in.close();
                        clientSocket.close();
                        serverSocket.close();
                        break;
                    }


                    Thread.sleep(5000);
                }
            }
            catch(Exception e)
            {
                System.err.println(System.currentTimeMillis()+" Unexpected Error");
            }
        }
    });
    t.start();
}
}

and the Client class

/* Client.java */
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;

public class Client
{
private static final int PORT = 50000;
private static final String HOST = "localhost";

public static void main(String[] args) throws IOException
{
    Socket socket = null;

    try
    {
        socket = new Socket(HOST, PORT);
    }
    catch(Exception e)
    {
        System.err.println("Could not connect to "+HOST+":"+PORT);
        System.exit(1);
    }

    final PrintWriter out = new PrintWriter(socket.getOutputStream(),true);
    final BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

    Thread t = new Thread(new Runnable()
    {
        public void run()
        {
            long start = System.currentTimeMillis();

            while (true)
            {
                try
                {
                    String input = in.readLine();

                    if (input != null)
                    {
                        System.out.println(System.currentTimeMillis() + " Server: " + input);
                    }

                    if (input.equals("Ping"))
                    {
                        if(System.currentTimeMillis()-start>30000)
                        {
                            out.println("Pon g");
                            System.out.println(System.currentTimeMillis() + " Client: Pon g");
                            break;
                        }

                        out.println("Pong");
                        System.out.println(System.currentTimeMillis() + " Client: Pong");
                    }
                }
                catch (IOException ioe)
                {
                    //
                }
            }
        }
    });
    t.start();

    out.close();
    in.close();
    socket.close();
}
}

Here is the output on running

Wating for connection............
Client connected on port 50000
1368986914928 Ping sent
java.lang.NullPointerException
    at Server$2.run(Server.java:84)
    at java.lang.Thread.run(Thread.java:722)