Add method for Collection.
Add a fail for null-worlds on updating set-back locations internally.

Setting a set-back location with a null World will now lead to a
NullPointerException. Setting null worlds on set-back locations is not
intended by contract, however due to using temporary Location instances
with Entity.getLocation(Location) and setting the world to null
afterwards to prevent memory leaks, also considering the number of
places where setSetBack is used, this seems to be an appropriate measure
to track better or help excluding violations of that contract.