Erreur Java !!

Exception in thread "main" java.lang.NullPointerException
        at Cell.setIcon(App.java:30)
        at Cell.<init>(App.java:25)
        at App.<init>(App.java:46)
        at App.main(App.java:58)

import java.awt.Color;
import java.awt.GridLayout;
import java.awt.event.*;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;

class Cell extends JButton {

    private static final int NUM_COLOURS = 3;

    private ImageIcon[] icons;
    private int type;

    public Cell() {
 ImageIcon[] icons;

 icons = new ImageIcon[NUM_COLOURS];
 for (int i=0; i<NUM_COLOURS; i++) {
     icons[i] = new ImageIcon("data/ball-" + Integer.toString(type) + ".png");
 }
 
     this.type = 0;
     setBackground(Color.WHITE);
     setIcon();
     setBorderPainted(false);
    }

    private void setIcon() {
 setIcon(icons[type]);
    }

    public void update() {
     type = type + 1;
     setIcon();
    }
    
}

public class App extends JFrame implements ActionListener {

    private Cell myCell;
    
    public App() {
 setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 myCell = new Cell();
 myCell.addActionListener(this);
 add(myCell);
 pack();
 setVisible(true);
    }

    public void actionPerformed(ActionEvent e) {
 myCell.update();
    }
    
    public static void main(String[] args) {
 new App();
    }
    
}