This error means that the world couldn't be initialised because of an error in the world. If you post the code of the world we can tell you whats wrong with it.

import greenfoot.*; // (World, Actor, GreenfootImage, and Greenfoot) /** * A class that has a scrolling background image * * @author avik */ public class ScrollWorld extends World { private static final GreenfootImage bgImage = new GreenfootImage("balloon.png"); private static final int scrollSpeed = 1; private GreenfootImage scrollingImage; private int scrollPosition = 0; ScrollWorld() { super(800, 400, 1); GreenfootImage background = new GreenfootImage(800, 400); scrollingImage = getScrollingImage(800, 400); background.drawImage(scrollingImage, 0, 0); setBackground(background); addObject(new balloon(), 350, 200); } public void act() { if(scrollSpeed > 0 && scrollPosition <= 0) { scrollPosition = getWidth(); } if(scrollSpeed < 0 && scrollPosition >= getWidth()) { scrollPosition = 0; } scrollPosition -= scrollSpeed; paint(scrollPosition); } /** * Paint scrolling image at given position and make sure the rest of * the background is also painted with the same image. */ private void paint(int position) { GreenfootImage bg = getBackground(); bg.drawImage(scrollingImage, position, 0); bg.drawImage(scrollingImage, position - scrollingImage.getWidth(), 0); } /** * Returns an image with the given dimensions. */ private GreenfootImage getScrollingImage(int width, int height) { GreenfootImage image = new GreenfootImage(width, height); for(int x = 0; x < width; x += bgImage.getWidth()) { for(int y = 0; y < height; y += bgImage.getHeight()) { image.drawImage(bgImage, x, y); } } return image; } }