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.

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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
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stack frame is
gml_Object_oBase_Step_0 (line -1)