I've gotten this crash multiple times when destroying various entities. First time was a boompuff on Gleba, second time was an assembler on Vulcanus.

Error while running event multi-rocket-part-recipes::on_object_destroyed (ID 174)
multi-rocket-part-recipes/control.lua:53: table index is nil
stack traceback:
multi-rocket-part-recipes/control.lua:53: in function <multi-rocket-part-recipes/control.lua:50>

what other mods are you using?
I can't think of any reason why that would happen

I'm running with a lot of other mods and I haven't been able to reproduce it yet when starting a new game with a reduced set of mods. I haven't had much time to debug the issue so I wrapped the crashing lines in an if just to make it go away:

    if unit_number then
        storage.rocket_silos[unit_number] = nil
        storage.rocket_parts[unit_number] = nil
        storage.rocket_silo_rocket_parts_per_craft[unit_number] = nil
    end

So far that's been working beautifully.

Today when looking into it further, it happened every time I mined an inserter. So far I've reproduced the crash with fast, bulk, and stack inserters.

I think I've narrowed it down to an interaction with one of the following 3 mods:
Inserter Fuel Leech
Inserter Drop Lane
Filter Helper

The tricky part is, even after disabling those mods, any inserter I placed while they were active will still crash Factorio. Only newly placed inserters are "fixed".

I think I've narrowed it down to Inserter Fuel Leech.