Help!! my score.....

import greenfoot.*;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Font;


public class Counter extends Actor
{
    private static final Color textColor = new Color(0, 0, 0);
    
    private int value = 0;
    private int target = 0;
    private String text;
    private int stringLength;
    private Font textFont = new Font ("Arial", Font.BOLD, 14);

    public Counter()
    {
        this("");
    }

    public Counter(String prefix)
    {
        text = prefix;
        stringLength = (text.length() + 2) * 10;

        setImage(new GreenfootImage(stringLength, 16));
        GreenfootImage image = getImage();
        image.setColor(textColor);
        image.setFont(textFont);
        updateImage();
    }
    
    public void act() {
        if(value < target) {
            value++;
            updateImage();
        }
        else if(value > target) {
            value--;
            updateImage();
        }
    }

    public void add(int score)
    {
        target += score;
    }

    public void subtract(int score)
    {
        target -= score;
    }

    public int getValue()
    {
        return value;
    }

    private void updateImage()
    {
        GreenfootImage image = getImage();
        image.clear();
        image.drawString(text + value, 1, 12);
    }
}

Bullet = getOneIntersectingObject(Bullet.class);

Counter counter = new Counter("Score"); //Put whatever you want instead of "Score", just make sure it has quotes around it  

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 Counter extends Actor
{
    private static final Color transparent = new Color(0,0,0,0);
    private GreenfootImage background;
    private int value;
    private int target;

    /**
     * Create a new counter, initialised to 0.
     */
    public Counter()
    {
        background = getImage();  // get image from class
        value = 0;
        target = 0;
        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("" + value, 22, Color.BLACK, transparent);
        image.drawImage(text, (image.getWidth()-text.getWidth())/2, 
                        (image.getHeight()-text.getHeight())/2);
        setImage(image);
    }
}

public class Space extends World
{

    /**
     * Constructor for objects of class Space.
     * 
     */
    public Space()
    {    
        // Create a new world with 800x600 cells with a cell size of 1x1 pixels.
        super(800, 600, 1); 

        prepare();
    }

    /**
     * Prepare the world for the start of the program. That is: create the initial
     * objects and add them to the world.
     */
    private void prepare()
    {              
        RestartSun restartsun2 = new RestartSun();
        RestartSun restartsun3 = new RestartSun();
        addObject(restartsun3, 799, 41);
        MenuMoon menumoon2 = new MenuMoon();
        addObject(menumoon2, 21, 8);
        Rocket rocket = new Rocket();
        addObject(rocket, 68, 218);
        Rock rock = new Rock();
        addObject(rock, 567, 374);
        Rock rock2 = new Rock();
        addObject(rock2, 466, 222);
        Rock rock3 = new Rock();
        addObject(rock3, 428, 45);
        Rock rock4 = new Rock();
        addObject(rock4, 80, 338);
        Rock rock5 = new Rock();
        addObject(rock5, 228, 290);
        Rock rock6 = new Rock();
        addObject(rock6, 130, 58);
        Rock rock7 = new Rock();
        addObject(rock7, 388, 337);
        rocket.setLocation(94, 189);
        rock4.setLocation(91, 525);
        rock5.setLocation(301, 488);
        rock7.setLocation(320, 383);
        rock2.setLocation(540, 509);
        rock.setLocation(738, 453);
        rock2.setLocation(573, 527);
        Rock rock8 = new Rock();
        addObject(rock8, 652, 165);
        rock7.setLocation(484, 373);
        Rock rock9 = new Rock();
        addObject(rock9, 205, 355);
        Rock rock10 = new Rock();
        addObject(rock10, 317, 143);
        rock9.setLocation(177, 365);
        rock9.setLocation(157, 348);
        rock10.setLocation(313, 139);
        rocket.setLocation(74, 284);
        rock9.setLocation(182, 397);
        rocket.setLocation(63, 302);
        Bullet bullet = new Bullet();
        addObject(bullet, 137, 301);
        removeObject(bullet);
        Counter counter7 = new Counter("0");
        addObject(counter7, 65, 582);
        counter7.setLocation(56, 582);

Counter counter = (Counter)getWorld().getObjects(Counter.class).get(0);

((Counter)getWorld().getObjects(Counter.class).get(0)).add(/* amount to add */);

 Actor Bullet;
    Bullet = getOneObjectAtOffset(0, 0, Bullet.class);
    if (Bullet != null)
    {
        World world;
        world = getWorld();
        world.removeObject(this);
        world.removeObject(Bullet);
        Counter counter = (Counter)getWorld().getObjects(Counter.class).get(0);
        counter.add(10);