Sorry for the (extremely long) delay in getting to this; the way my mod works is by looking at the next_upgrade field for the machines in question, in order to locate the next machine it should copy for the following micro-assembler.
It seems that 248k mod adds something called "fi_crafter_entity" to the next_upgrade field for assembler-3, which implies that there should be a micro-assembler-4, and would attempt to copy that, but in vanilla, my mod stops at 3, so it sets up the next_upgrade, without actually creating micro-assembler-4
EDIT: It looks like this "fi_crafter_entity" corresponds to the nuclear assembler, which apparently is the final upgrade of the assembler line, despite requiring nuclear fuel instead of electricity, so feels like it should be a side-grade to me, especially since it doesn't visually match the assemblers it is intended to replace.
But either way, two solutions; I can make a micro-assembler-4, using the nuclear assembler as a base, though it would have the wrong name, or I could simply write the next_upgrade as nil, and the error should disappear. I think I'll go with the latter for now.
