nullPointerException

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

/**
 * Write a description of class Controls here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Controls extends Actor
{
    private GreenfootImage image1=new GreenfootImage("MenuButtonControls.png");
    private GreenfootImage image2=new GreenfootImage("MenuButtonControlsOutline.png");
    public Controls()
    {
        setImage(image1);
    }
    /**
     * Act - do whatever the Controls wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        changeImage();
    }  
    private void changeImage()
    {
        Menu menu=(Menu)getWorld();
        MouseInfo mouse = Greenfoot.getMouseInfo();
        if(mouse.getX()>getX()-150&&mouse.getX()<getX()+150&&mouse.getY()<getY()+75&&mouse.getY()>getY()-75)
        {
            setImage(image2);
        }
        else
        {
            setImage(image1);
        }
    }
}