Crash on Battle Start

I’d been playing for an hour when I got a crash on a battle start:

Version: Windows 10 via Steam @ version 1.3.20


############################################################################################
FATAL ERROR in
action number 1
of Create Event
for object obj_battlecontroller:

Variable obj_battlecontroller.xp(100223, -2147483648) not set before reading it.
at gml_Object_obj_battlecontroller_Create_0
############################################################################################

stack frame is
gml_Object_obj_battlecontroller_Create_0 (line -1)
called from - gml_Script_instance_create (line -1) -
gml_Room_battle_Create