nullPointer

import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
import java.awt.Color;
import java.util.List;
/**
 * Write a description of class MyWorld here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Ocean extends World
{
    private int counter=39;
    private int counter2=650;
    private int counter3=0;
    private int ident=0;
    private Oxygen[] oxygen=new Oxygen[50];
    /**
     * Constructor for objects of class MyWorld.
     * 
     */
    public Ocean()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(640, 600, 1,false); 
        getBackground().setColor(new Color(0,186,255));
        getBackground().fillRect(0,(getHeight()/3)+13,getWidth(),getHeight());
        getBackground().setColor(new Color(133,228,224));
        getBackground().fillRect(0,0,getWidth(),(getHeight()/3)-12);
        setPaintOrder(Dolphin.class);
        drawBackgroundPics();
        setup();
    }

    /**
     * The main method.
     */
    public void act()
    {
        oxygen();
        addWave();
        addCloud();
    }

    /**
     * Set up the beginning actors.
     */
    private void setup()
    {
        addObject(new Cloud(),150,120);
        addObject(new Cloud(),500,55);
        for(int i=40;i<getWidth();i+=80)
        {
            addObject(new Wave(-2),i,getHeight()/3);
        }
        for(int i=0;i<50;i++)
        {
            addObject(oxygen[i],501+(i*2),566);
        }
    }

    /**
     * Add a wave object in certain increments.
     */
    private void addWave()
    {
        counter++;
        if(counter==40)
        {
            counter=0;
            addObject(new Wave(-2),getWidth()+40,getHeight()/3);
        }
    }

    /**
     * Add a cloud object in certain increments.
     */
    private void addCloud()
    {
        counter2+=5;
        if(counter2==700)
        {
            addObject(new Cloud(),850,120);
            counter2=0;
        }
        if(counter2==350)addObject(new Cloud(),850,55);
    }

    /**
     * Draw backgound pictures. 
     */
    private void drawBackgroundPics()
    {
        for(int i=0;i<665;i+=166)
        {
            getBackground().drawImage(new GreenfootImage("Mud.png"),i,getHeight()-84);
        }
        getBackground().drawImage(new GreenfootImage("rock2.png"),75,getHeight()-100);
        getBackground().drawImage(new GreenfootImage("grass.png"),475,getHeight()-115);
        getBackground().setColor(new Color(255,255,28));
        getBackground().fillOval(-50,-50,100,100);
        getBackground().setColor(Color.black);
        getBackground().drawRect(500,550,101,31);
    }

    private void oxygen()
    {
        // counter3++;
        int num=49;
        List<Oxygen>oxygen=getObjects(Oxygen.class);
        Oxygen ox;
        for(int i=0;i<oxygen.size();i++)
        {
            if(oxygen.get(i).getIdentity()==num)
            {
                removeObject(oxygen.get(i));
                
                //break ;
            }
            num--;
        }

        /*if(counter3==2)
        {
        for(Oxygen o2:oxygen)
        {
        if(o2.getIdentity()==ident)
        {
        removeObject(o2);
        ident--;
        break;
        }
        }

        counter3=0;
        }*/
    }
}

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

/**
 * Write a description of class Oxygen here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Oxygen extends Actor
{
    private int identifier;
    public Oxygen(int num)
    {
        setImage("Oxygen-bar.png");
        identifier=num;
    }

    public Oxygen()
    {
        setImage("Oxygen-bar.png");
    }

    /**
     * Act - do whatever the Oxygen wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        // Add your action code here.
    }    

    public int getIdentity()
    {
        return identifier;
    }
}

java.lang.NullPointerException
	at greenfoot.World.addObject(World.java:396)
	at Ocean.setup(Ocean.java:57)
	at Ocean.<init>(Ocean.java:31)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
	at greenfoot.core.Simulation.newInstance(Simulation.java:617)
	at greenfoot.platforms.ide.WorldHandlerDelegateIDE$3.run(WorldHandlerDelegateIDE.java:425)
	at greenfoot.core.Simulation.runQueuedTasks(Simulation.java:502)
	at greenfoot.core.Simulation.maybePause(Simulation.java:305)
	at greenfoot.core.Simulation.runContent(Simulation.java:218)
	at greenfoot.core.Simulation.run(Simulation.java:211)