Change line 61 back. The move should always be value as any adjacent valid location to the empty location will have a mover. With the way the offsets were arrived at, no invalid locations will be among them. Also, collision checking fails during the 'for' loop. That is why I rearranged the parts of the code -- to avoid collision checking when not needed.

"Act" mthod in world? I didn't know this was a thing, I thought that "if" from the constructor will do the work if I add Greenfoot.stop() alongside with the end photo. How does that work?

Yes -- act method in the world. The act method is for what an actor needs to repeatedly do; the act method of the world is for what needs to be done repeatedly by the world. General game play codes usually go within the world. The continuous checking for game over is not a behavior of any actor; it is something that the world should perform. Just add an 'act' method (just like it appears in an Actor subclass) to the world class code and check for game over in it. The constructor is only executed once, when the object is being created. You cannot put any code that needs to continuously execute in it.