Crash in arena

Got the following script after a crash when doing arena. Unsure what all info would be useful here but it occurred after my djinn pyromancer attacked the last creature in the round


############################################################################################
FATAL ERROR in
action number 1
of Step Event0
for object obj_battlecontroller:

Variable obj_battlecontroller.slot(100063, -2147483648) not set before reading it.
at gml_Script_bc_CreatureAttack
############################################################################################

stack frame is
gml_Script_bc_CreatureAttack (line -1)
gml_Script_bc_OnDeath
gml_Script_bc_ExecuteEvents
gml_Object_obj_battlecontroller_Step_0