Not really sure what caused it. Playing in endless mode maybe 15 minutes in. Had played several Classic mode games for several hours with no problem.
############################################################################################
FATAL ERROR in
action number 1
of Step Event0
for object oBase:
local variable build_obj(100004, -2147483648) not set before reading it.
at gml_Object_oBase_Step_0
############################################################################################
--------------------------------------------------------------------------------------------
stack frame is
gml_Object_oBase_Step_0 (line -1)