Fixed NullPointerException in yCoordHelper

I believe I've fixed the cause of a NullPointerException that some
players have been experiencing. It seems to have been caused by
unregistered block IDs being present in Limbo. Possibly because people
removed mods and they had generated structures in Limbo. Or the players
had placed blocks and then removed the mods, or changed block IDs
around.

This fixes issue #47.