Callback fail on IconListener

Question

I am trying to get the hang of using callback in relation to several clients receiving Icons from a server.

My code so far looks like this. Can anyone please point me in the right direction. I get a NullPointerException when i try to use the IconListener arraylist.

package p2;

import java.awt.Dimension;
import java.awt.FlowLayout;

import javax.swing.Icon;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class P2Viewer extends JPanel {
    private JLabel lblIcon = new JLabel();
    private IconClient iconClient;
    private IconManager iconManager;

    public P2Viewer(int width, int height) {
        setLayout(new FlowLayout(FlowLayout.CENTER));
        lblIcon.setOpaque(true);
        add(lblIcon);
        setPreferredSize(new Dimension(width, height));
    }

    public P2Viewer(IconClient iconClient, int width, int height) {
        this(width, height);
        this.iconClient = iconClient;
        this.iconClient.addIconListener(new Listener());

    }

    public void setIcon(Icon icon) {
        lblIcon.setIcon(icon);
    }

    private class Listener implements IconListener {
        @Override
        public void Icon(Icon icon) {
            if (icon instanceof Icon) {
                setIcon(icon);
            }

        }

    }
}

package p2;

import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.net.Socket;
import java.util.ArrayList;

import javax.swing.Icon;

public class IconClient extends Thread {

    private Socket socket;
    private ObjectInputStream ois;
    private Icon icon;
    private ArrayList<IconListener> iconlistener;

    public IconClient(String ip, int port) {
        try{
            socket = new Socket(ip, port);
            ois = new ObjectInputStream(socket.getInputStream());
            start();
        }catch(IOException e){
            System.err.println(e);
        }
    }

    public void addIconListener(IconListener iconListener) {
        this.iconlistener.add(iconListener);
    }

    public void run() {
        while (true) {
            try {
                icon = (Icon) ois.readObject();
                System.out.println("Icon received");
                for(IconListener iconList:iconlistener){
                    iconList.Icon(icon);
                }
                System.out.println("Icon sent to listener");
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}