Fix HouseRaffleStone timer cleanup

[Alcsaar]

Summary

Fixes a NullReferenceException when deleting/removing the last active HouseRaffleStone.

AddRaffleStone() started the shared repeating raffle timer but did not assign it to _allStonesTimer. When the final tracked raffle stone was removed, RemoveRaffleStone() attempted to call _allStonesTimer.Stop(), causing a null reference.

Changes

• Store the shared raffle timer when it is created.
• Avoid duplicate timer registration for the same stone.
• Stop the shared timer null-safely when the final tracked stone is removed.
• Guard the timer tick against a null stone set.

Testing

• Tested removing stone in game during active raffles / etc to ensure no further DCs