Java getting serializer error with kryonet and slick2d?

[rokyslash]

I am trying to execute this server code.

package server;

import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.io.IOException;

import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;

import org.apache.logging.log4j.message.Message;

import com.esotericsoftware.kryo.Kryo;
import com.esotericsoftware.kryonet.Server;
import com.esotericsoftware.minlog.Log;

import Client.PlayerChar;
import NetworkClasses.LoginRequest;
import NetworkClasses.LoginResponse;

public class MainServer {

    private int tcpPort;
    private int udpPort;
    public static Server server;
    private Kryo kryo;

    static JFrame jFrame;
    static JTextArea jTextArea;
    static MainServerListener listener = new MainServerListener();

    public MainServer(int tcpPort, int udpPort) {
        this.tcpPort = tcpPort;
        this.udpPort = udpPort;
        server = new Server();

        kryo = server.getKryo();
        registerKryoClasses();
    }

    public void startServer() {
        Log.info("Starting Server");
        jTextArea.append("Starting Server...");
        jTextArea.append("\n");
        server.start();
        try {
            server.bind(tcpPort, udpPort);
            server.addListener(listener);
            jTextArea.append("Server online! \n");
            jTextArea.append("----------------------------");
            jTextArea.append("\n");
            update();
        } catch (IOException e) {
            Log.info("Port already used");
            jTextArea.append("Port already in use");
            jTextArea.append("\n");
            e.printStackTrace();
        }
    }

    // Try changing this to non staic and see where this effects our game
    public static void stopServer() {
        Log.info("Server stopped");
        jTextArea.append("Server stopped.");
        jTextArea.append("\n");
        server.stop();
    }

    public void update() {
        while (true) {

        }
    }

    private void registerKryoClasses() {
        kryo.register(LoginRequest.class);
        kryo.register(LoginResponse.class);
        kryo.register(Message.class);
        kryo.register(PlayerChar.class);
        kryo.register(org.newdawn.slick.geom.Rectangle.class);
        kryo.register(float[].class);
        kryo.register(NetworkClasses.PacketUpdateX.class);
        kryo.register(NetworkClasses.PacketUpdateY.class);
        kryo.register(NetworkClasses.PacketAddPlayer.class);
        kryo.register(NetworkClasses.PacketRemovePlayer.class);

    }

    public static void createServerInterface() {
        jFrame = new JFrame("GameServerInterface");
        jTextArea = new JTextArea();
        jTextArea.append("\n");
        jTextArea.setLineWrap(true);
        jTextArea.setEditable(false);

        jFrame.add(jTextArea);
        jFrame.setSize(400, 600);
        jFrame.addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                int i = JOptionPane.showConfirmDialog(null, "You want to shut down the server?");
                if(i == 0) {
                    stopServer();
                    System.exit(0); // successful exit
                }
            }
        });

        JScrollPane scrollPane = new JScrollPane(jTextArea);
        scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
        scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        jFrame.add(scrollPane);
    }

    public static void main(String args[]) {
        Log.set(Log.LEVEL_INFO);

        MainServer main = new MainServer(55555, 55556);
        createServerInterface();
        main.startServer();
    }

} // end total class

Full source code is at https://github.com/schepan1/ms/tree/master/src/main/java. However when I try to run this server, I keep getting this error/exception and there are no solutions online at all. How can I solve this?

Exception in thread "main" java.lang.IllegalArgumentException: Unable to create serializer "com.esotericsoftware.kryo.serializers.FieldSerializer" for class: com.esotericsoftware.kryonet.FrameworkMessage$RegisterTCP
    at com.esotericsoftware.kryo.Kryo.newSerializer(Kryo.java:337)
    at com.esotericsoftware.kryo.Kryo.newDefaultSerializer(Kryo.java:316)
    at com.esotericsoftware.kryo.Kryo.getDefaultSerializer(Kryo.java:309)
    at com.esotericsoftware.kryo.Kryo.register(Kryo.java:353)
    at com.esotericsoftware.kryonet.KryoSerialization.<init>(KryoSerialization.java:33)
    at com.esotericsoftware.kryonet.KryoSerialization.<init>(KryoSerialization.java:25)
    at com.esotericsoftware.kryonet.Server.<init>(Server.java:91)
    at com.esotericsoftware.kryonet.Server.<init>(Server.java:73)
    at server.MainServer.<init>(MainServer.java:36)
    at server.MainServer.main(MainServer.java:118)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.esotericsoftware.kryo.Kryo.newSerializer(Kryo.java:324)
    ... 9 more
Caused by: java.lang.IncompatibleClassChangeError: Found interface org.objectweb.asm.MethodVisitor, but class was expected
    at com.esotericsoftware.reflectasm.FieldAccess.insertConstructor(FieldAccess.java:144)
    at com.esotericsoftware.reflectasm.FieldAccess.get(FieldAccess.java:109)
    at com.esotericsoftware.kryo.serializers.FieldSerializer.rebuildCachedFields(FieldSerializer.java:104)
    at com.esotericsoftware.kryo.serializers.FieldSerializer.<init>(FieldSerializer.java:50)
    ... 14 more