java.lang.IllegalStateException: Actor not in world. An attempt was made to use the actor's location while it is not in the world. Either it has not yet been inserted, or it has been removed.
	at greenfoot.Actor.failIfNotInWorld(Actor.java:681)
	at greenfoot.Actor.getOneObjectAtOffset(Actor.java:885)
	at Roboter.act(Roboter.java:39)
	at greenfoot.core.Simulation.actActor(Simulation.java:594)
	at greenfoot.core.Simulation.runOneLoop(Simulation.java:552)
	at greenfoot.core.Simulation.runContent(Simulation.java:215)
	at greenfoot.core.Simulation.run(Simulation.java:205)

import greenfoot.*;
import java.awt.Color;
/**
 * Write a description of class Roboter here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class Roboter extends Actor
{
    public int mouseX, mouseY;
    
    public int personRotation;
    int angle = getRotation();
    
    
    /**
     * Act - do whatever the Roboter wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        hitRock();
        mouse();
        if(Greenfoot.isKeyDown("W"))
        {
            move(3);
            

        }
        if(Greenfoot.isKeyDown("S"))
        {
            move(-3);
            

        }
    
        Actor mineral;
        mineral = getOneObjectAtOffset (0, 0, Mineral.class);
        if(mineral != null)
        {
           World myWorld = getWorld();
           Surface surface = (Surface)myWorld; 
           Counter counter = surface.getCounter();
           counter.addScore();
           surface.removeObject(mineral);
           return;
        }
       }    public void hitRock()
    {
      {
        Actor rock = getOneIntersectingObject(Rock.class);
        if(rock != null)
        {
            World myWorld = getWorld();
            GameOver gameover = new GameOver();
            myWorld.addObject(gameover, myWorld.getWidth()/2, myWorld.getHeight()/2);
            myWorld.removeObject(this);
            
        }
      }
    }
    public void mouse()
    {
        if(Greenfoot.getMouseInfo() !=null)
        {
            mouseX = Greenfoot.getMouseInfo().getX();
            mouseY = Greenfoot.getMouseInfo().getY();

            turnTowards(mouseX, mouseY);
            
        }
    }
    
    }