I did it by putting everything in the act method to a 'everything()' method then I did if else statement (this != null), if part is time is ticking, else part is it is not. So it is working. I had some issues on printing the time in the middle on the scoreboard and removing the other one but now I figured it out. But...
I have another problem actually not a problem but a improvement, so I have 5 different enemy planes in the game and I used this for giving numbers to them.
But now I wanted do them increasing a little for example I want the constant number '20' in this case to increase 5 or 10 in each other airplane. How can I do that?
public int randomValueAirplane2 = 20+Greenfoot.getRandomNumber(30);