The problem is, that even though you removed the knight from the display list, the code is still running in the onTick function, and you are trying to access something that is not existing, I suggest you to check whether your knight's reference is null or not, if not, then run the code, else skip it.
EDIT:
Whenever you null an instance (myInstance = null
), you can no longer access to it's properties, functions etc.. and if you try, then you get that error above. To avoid it, check whether your object is null or not.
if(myInstance != null) {
//do your stuff with your instance
}