Just posting it because it might help ... wasnt really doing anything specific at the time, but it has happened consistently a few minutes after I open my save.

Error while running event beltlayer::on_robot_built_entity (ID 14)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
beltlayer/Blueprint.lua:114: in function 'abort_robot_build'
beltlayer/Blueprint.lua:281: in function 'on_robot_built_entity'
beltlayer/control.lua:32: in function <beltlayer/control.lua:28>

It looks like there's a problem when a robot tries to place an underground belt on its ghost, but the location is obstructed underground, and there's a surface-level belt in the same tile (overlapping the ghost). So there are 4 "belts" in the same place: a real belt on the surface, a real belt underground, a ghost for an underground belt on the surface, and a belt carried by the robot to try to build the ghost.

It shouldn't be possible to get into this situation in 0.1.6, but you may still have some confused robots in-flight that are causing your crash. I'll have to work on a release that removes existing ghosts that were somehow created in a previous version of the mod.