Crash when replacing Lorian with Carthus Sandworm #14
Description
This came up when a player was using a custom preset with the Archipelago randomizer, but I expect it would reproduce in the standalone randomizer as well. Debugging into it, it looks like when fromEntity is Lorian and toEntity is Carthus Sandworm, this code ends up with arena == null and expectArena == false. It then reaches this line which crashes with a null pointer exception.
From user reports there may be other enemy combinations that produce this, but I wasn't able to determine what they were.