This sometimes happens when the ship gets launched (probably during its cloning somewhere else?).
Most of the time this does not happen, but then suddenly it occurs.

The mod Space Exploration (0.5.77) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event space-exploration::on_tick (ID 0)
The mod Utilization Monitor (Blargh) (0.8.7) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event UtilizationMonitorBlargh::on_entity_cloned (ID 121)
UtilizationMonitorBlargh/control.lua:526: attempt to index field '?' (a nil value)
stack traceback:
UtilizationMonitorBlargh/control.lua:526: in function <UtilizationMonitorBlargh/control.lua:519>
stack traceback:
[C]: in function 'clone_brush'
space-exploration/scripts/spaceship-clone.lua:231: in function 'clone'
space-exploration/scripts/spaceship.lua:579: in function 'launch'
space-exploration/scripts/spaceship.lua:2289: in function 'stop_integrity_check'
space-exploration/scripts/spaceship.lua:2611: in function 'integrity_check_tick'
space-exploration/scripts/spaceship.lua:1555: in function 'spaceship_tick'
space-exploration/scripts/spaceship.lua:1850: in function 'callback'
space-exploration/scripts/event.lua:15: in function <space-exploration/scripts/event.lua:13>

the /umreset command helps to avoid this condition

That's interesting - I have Space Exploration as one of the mods I test against and I haven't seen this happen. I'll take a look.

I did some cleanup of that clone code in 0.8.8. Give that version a shot and see if you still see the issue? I wasn't able to reproduce it locally.

Another person got me a save file and reproduction steps for the old 0.8.7 version, and 0.8.8 does fix that reproduction. Hopefully you'll be good. Let me know, and thanks for the report!

Thank you for such a quick response!
I've updated the mod and will give it a go to see if I get the error in the future.

Thank you for your awesome mod, I really love it!