Actually, a few frames up the stack trace is AdventureResult.getCount( inventory ), and inventory contains AdventureResults, so the Candy comparator shouldn't be relevant to this particular crash.
The weird thing about xKiv's comment #3 (which is where I arrived as well) about something in inventory being null is that SortedListModel.add(int, E) explicitly refuses to add null elements. It's possible that somewhere is calling LockableListModel.set(null). I couldn't find any errors when I skimmed SortedListModel's binary search implementation, but that's also a possible culprit.
(The only instance of inventory.set( index, result ) I could find is in AdventureResult.addResultToList(), but that looks to be adequately guarded against setting the value to null -- for one, indexOf would return -1 in that case, and we'd just call add().)
(tangent: could we rip out the SortedListModels in favor of something more standard, like ConcurrentHashMaps if we actually need locking? I'm inclined to trust that said implementations have been much more heavily vetted by way of much wider industry adoption.)