Cannot call method 'setPosition' of null.

[cjsaylor]

Welcome .... prepare to be attacked!
Round 1
Player health: 1000

Round 2
Player health: 1000
Targeting 1 with power attack mode.
... barely scratches grunt(1). (2 damage)
grunt (1) moved to position 3
https://node-defender.herokuapp.com/js/game_board.js156: Uncaught TypeError: Cannot call method 'setPosition' of null

Happens periodically but is resolved by refreshing the page. Could be a race condition moving an enemy that is no longer in the set.