Don't you just love NullReferenceExceptions :)

[bleedo]

Attached save and a nice screenshot to boot.

Don't envy you debugging that one.

Plain A3.410, no mods.

[attachment deleted by admin: too old]

[WorldOfIllusion]

your save names are excellent.

[ItchyFlea]

Vanilla RimWorld or were you using mods? If using mods, which ones, and in what order were they activated?

[bleedo]

Vanilla. Edited, thanks.

[ItchyFlea]

This should be interesting to figure out. No promises. 

EDIT: Fixed save attached below.
The two faction squads that were present when that game was saved have been removed. The problem was caused by one of them.

Thank-you for providing the save. It will help us find the exact cause of the bug, and hopefully lead to a fix that will prevent it from occurring in the future.
The problem is caused by loading a save that has pawns from a friendly faction providing military assistance present in the world.
A workaround until this is fixed is to save the game prior to calling another faction for military assistance, or to save after they have left.

EDIT3: Definitely a bug. Reported on Mantis.

[attachment deleted by admin: too old]

[Architect]

Were you using the hotkeys in the architect to build something at the time? Or is this from loading up a save game?

[bleedo]

Thanks for making it proper.