Unity crashes whenever I try to modify a prefab

Trying to revive a project that I worked on in 2015, upgraded from Unity 5.0.0f4 to the very latest version. I've managed to get the game up-and-running, barring some references to old .js scripts. However, whenever I try to edit a prefab, Unity crashes when I save, or even just modify the prefab. I'm on Unity 2018.3, doing something as trivial as adding an empty gameobject to the simplest prefab I have produces a window with the unity logo and a red exclamation mark, followed by a crash. The prefab: https://pastebin.com/AnKpiJmz And the logic: https://pastebin.com/4WM2fccq I imagine the prefab data is somehow incompatible with the new editor - how can I fix this?