No. It is not like a normal error that appears in the bottom line of the classes window. Instead it is in it's own window, a small one that exactly says: It was not possible to instantiate a new world. The reason for this is usually that the world does not have a public default constructor (a constructor that takes no parameters) or that the world is not a public class. I've checked all the worlds but nothing was private, so I guess it is the "no parameters"

Actually, you do not need both constructors in the 'welt' class; just the one that begins with 'public welt(int img)'. You need to right-click on the Game icon and select 'new Game()' to make that the starting world.