Frontend does not respond when Multicasting

Question

I'm programming a multiplayer game in java and want to see available servers in the current network by doing a multicast. The problem is that as soon as I start the Multicast Server the Frontend stops responding and I'm forced to close the window. I'm using JavaFX.

Anyone with an idea?

Thanks in advance!

package server;

import backend.game.Game;

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;
import java.net.MulticastSocket;
import java.net.UnknownHostException;


// Multicast Server telling clients he is available for connection
public class MulticastReceiver extends Thread{
    protected MulticastSocket socket = null;
    protected byte[] buf = new byte[256];
    public InetAddress group;
    private Game game;

    //main for testing
    public static void main(String[] args) {
        try {
            MulticastReceiver mr = new MulticastReceiver(new Game(InetAddress.getLocalHost().getHostAddress(), "Milans test game"));
        } catch (UnknownHostException e) {
            e.printStackTrace();
        }

    }

    public MulticastReceiver(Game game){
        this.game = game;
        this.run();
    }

    @Override
    /**
     * @author mestorff
     * Thread listening for messages coming in on IP 230.0.0.0 and handeling them
     */
    public void run(){
        try {
            socket = new MulticastSocket(4446);

        group = InetAddress.getByName("230.0.0.0");
        socket.joinGroup(group);
        socket.setReuseAddress(true);

        while(!Thread.interrupted() && !socket.isClosed()){
            DatagramPacket packet = new DatagramPacket(buf,buf.length);
            socket.receive(packet);
            String received = new String(packet.getData(), 0, packet.getLength());
            //System.out.println(received);
            if("end".equals(received)){
                break;
            }
            String multicastAnswer = game.getGameID() + ":" + game.getGameName();
            byte[] bufAnswer = multicastAnswer.getBytes();

            DatagramPacket answerPacket = new DatagramPacket(bufAnswer, bufAnswer.length, packet.getAddress(), packet.getPort());
            socket.send(answerPacket);
            Thread.sleep(3000);
        }} catch (IOException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }

    /**
     * @author mestorff
     * Stop Thread and close socket
     */
    public void stopThread(){
        this.interrupt();
        try {
            socket.leaveGroup(group);
        } catch (IOException e) {
            e.printStackTrace();
        }
        socket.close();
    }