Replace line 125 with:
unless $scene.is_a?(Scene_Equip) or item.nil?
for a temporary fix, though I have no idea why Equip Extension would be calling a nil item. It's probably a logic error in either Pacman's or KGC's script.
I'm quite sure it was my script. I got an error in the gain_item method when the scene was Scene_Equip; for some reason the item wasn't retaining it's RPG class when it swapped, so I cunningly made sure it didn't happen in the equip scene. It didn't occur to me to put a .nil? there, that would've worked much better.
I still don't know why that would return nil, but that's my problem.