Got an error when overwhelmed by spitter venom.

Error while running event nice-try-train::on_pre_player_died(ID 40)
nice-try-train/control.lua:4: attempt to index field 'cause' (a nil Value) stack traceback:
nice-try-train/control.lua:4: in function
<nice-try-train/control.lua:1>

Not sure what caused this, since a train didn't hit me.

The cause is because the event is called just before a player dies, regardless of actual cause. In this case it appears that you were ultimately killed by venom that was still on the ground and does not have an associated cause for death (the death message would have just said "<player name> died" or something like that instead of "<player name> was killed by <cause of death>". This is indeed a bug and I've pushed out a fix.