at greenfoot.core.Simulation.run(Simulation.java:183)
java.lang.NullPointerException
	at Bullet.checkTankHit(Bullet.java:43)
	at Bullet.act(Bullet.java:21)
	at greenfoot.core.Simulation.actActor(Simulation.java:567)
	at greenfoot.core.Simulation.runOneLoop(Simulation.java:530)
	

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

/**
 * Write a description of class Bullet here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Bullet extends Actor
{
    private static final int damage =15;

    /**
     * Act - do whatever the Bullet wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        setLocation(getX(), getY()+1);
        explosion();
       if(getWorld()!=null)  checkTankHit();
        

    }    
    private void explosion()
    {
        if(isAtEdge()){
            getWorld().removeObject(this);
            
        }
    }

    /**
     * Check whether enemy bomb hit our tank or not
     */ 
    private void checkTankHit(){
        Tank tank =(Tank) getOneIntersectingObject(Tank.class);

        if (tank != null) 
        {
           getWorld().removeObject(this);
           Background background = (Background)getWorld();
           background.addScore(15);
    
        }
    }
}