I had a base defense previously, I'd say "two or three missions ago"....
In other words I'm sure I had other UFO recoveries since the base defense.
I haven't tinkered with the save whatsoever, so I can't tell of any other reason.
EDIT: I might add the base defense was NOT in the Base that's affected in this save, to memory this base hasn't yet been targeted by an alien assault. nevermind, read below.
EDIT2: I *guess* that toggling the property to False should take care of the crash, correct?
EDIT3: sorry I got things mixed up, reading the YAML files is a nightmare due to the wonky serialization order of yaml-cpp (or so I think) "Fire Island" is indeed the last base I had to defend.