nullPointerExeption

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

/**
 * Write a description of class Object here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Object extends Actor
{
    private int x;
    private int y;
    public Object()
    {
        
    }
    
    protected void addedToWorld(World world)
    {
    }
    
    /**
     * Act - do whatever the Object wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        setLocation(getX(),getY());       
    }   
    
    @Override
    public int getX()
    {
        ScrollActor actor=(ScrollActor)getOneIntersectingObject(ScrollActor.class);
        return Math.abs(actor.calcX()-super.getX());
    }
    
    @Override
    public int getY()
    {
        ScrollActor actor=(ScrollActor)getOneIntersectingObject(ScrollActor.class);
        return Math.abs(actor.calcY()-super.getY());
    }
}

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

/**
 * Write a description of class ScrollActor here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class ScrollActor extends Actor
{
    private int speed=10;
    public ScrollActor()
    {
        setImage("Scroll.jpg");
    }

    /**
     * Act - do whatever the ScrollActor wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        scrolling();
        getWorld().showText(""+getX(),250,235);
        getWorld().showText(""+getY(),250,265);
    }    
    
    /**
     * Tell the scrollActor where and when to move.
     */
    private void scrolling()
    {
        if(Greenfoot.isKeyDown("up")&&getY()<1245)setLocation(getX(),getY()+speed);
        if(Greenfoot.isKeyDown("down")&&getY()>-745)setLocation(getX(),getY()-speed);
        if(Greenfoot.isKeyDown("right")&&getX()>-745)setLocation(getX()-speed,getY());
        if(Greenfoot.isKeyDown("left")&&getX()<1245)setLocation(getX()+speed,getY());
    }
    
    /**
     * calculate the coord of the new 0 point on the x axis.
     */
    public int calcX()
    {
        return getX()-(getImage().getWidth()/2);
    }
    
    /**
     * calculate the coord of the new 0 point on the y axis.
     */
    public int calcY()
    {
        return getY()-(getImage().getHeight()/2);
    }
}

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

/**
 * Write a description of class Olam here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Olam extends World
{
    private ScrollActor scroller=new ScrollActor();
    /**
     * Constructor for objects of class Olam.
     * 
     */
    public Olam()
    {    
        super(500, 500, 1,false); 
        addObject(scroller,250,250);
        Greenfoot.start();
    }
}

java.lang.NullPointerException
	at Object.getX(Object.java:39)
	at Object.act(Object.java:29)
	at greenfoot.core.Simulation.actActor(Simulation.java:604)
	at greenfoot.core.Simulation.runOneLoop(Simulation.java:562)
	at greenfoot.core.Simulation.runContent(Simulation.java:221)
	at greenfoot.core.Simulation.run(Simulation.java:211)