This bug was reported on the RET thread here but it actually has to be addressed in Power Grid Comb. RET creates a "power-pole" entity named "ret-pole-wire", and if you delete it that messes up the mod. The solution is simply to exclude that type of pole from your optimization algorithm.

Simply changing control.lua line 7 to
if entity.type == "electric-pole" and entity.name ~= "ret-pole-wire" then
will achieve this

Thanks, I'll get a new version released with this compatibility fix!

That's super awesome, thanks, but is it possible to release the fix for 0.17?
I'm staying on stable as there are quite a few mods I'm using that haven't updated, and the list of improvements in 0.18 is relatively small (but growing...)