NullPointerException

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

/**
 * Write a description of class Bullet2 here.
 * 
 * @author Owain Jones
 * @version 1.1
 */

public class Bullet2 extends Cymeriadau
{
    private int direction, speed;
    private HealthDisplay health;
    
    public Bullet2(int dir)
    {
        direction = dir;
        speed = -15;
    }

    public void act()
    {
        setRotation(direction);
        move(speed);

        if( atWorldEdge() ) {
            getWorld().removeObject(this);
        }

        if (getWorld() != null) {
            hitPlayer1();
        }
    }

    public void hitPlayer1() {
        Player1 enemy = (Player1) getOneObjectAtOffset(0, 0, Player1.class);
        if (enemy != null) {
            health.setValue(-5);
            getWorld().removeObject(this);
        }

    }
}

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
import java.awt.Color;

/**
 * A simple counter with graphical representation as an actor on screen.
 * 
 * @author mik
 * @version 1.0
 */
public class HealthDisplay extends Actor
{
    private static final Color transparent = new Color(0,0,0,0);
    private GreenfootImage background;
    public static int value = 100;
    public static int target = 100;

    /**
     * Create a new counter, initialised to 0.
     */
    public HealthDisplay()
    {
        background = getImage();  // get image from class
        updateImage();
    }
    
    /**
     * Animate the display to count up (or down) to the current target value.
     */
    public void act() 
    {
        if (value < target) {
            value++;
            updateImage();
        }
        else if (value > target) {
            value--;
            updateImage();
        }
    }

    /**
     * Add a new score to the current counter value.
     */
    public void add(int score)
    {
        target += score;
    }

    /**
     * Return the current counter value.
     */
    public int getValue()
    {
        return value;
    }

    /**
     * Set a new counter value.
     */
    public void setValue(int newValue)
    {
        target = newValue;
        value = newValue;
        updateImage();
    }

    /**
     * Update the image on screen to show the current value.
     */
    private void updateImage()
    {
        GreenfootImage image = new GreenfootImage(background);
        GreenfootImage text = new GreenfootImage("Health:" + value, 22, Color.BLACK, transparent);
        image.drawImage(text, (image.getWidth()-text.getWidth())/2, 
                        (image.getHeight()-text.getHeight())/2);
        setImage(image);
    }
}