Nulref and disappearing contracts after (forcibly) satisfying a requirement

[Morphisor244]

This came to my attention after testing out an issue reported by user Friznit.
His issue is that a contract isn't satisfying its CompleteContract requirement, even though he did in fact complete the previous contract. I tried to reproduce this issue, but ran into the issues described below.

In both a fully stock install and a heavily modded and progressed career install (both ksp 1.10.1 with CC 1.30.5) I was unable to even properly test the issue, because after the game starts forcibly loading in the intended contract (after disabling the requirement), a red-text nullref exception is thrown, and the contract blips out of availability. A moment later it blips back in, errors and is gone again. I reproduced this for a whole range of contracts. Even upon accepting the contract as soon as it arrives, it gets auto-declined upon removal, citing that the requirements aren't met.

I'm 100% sure cheating the contracts into availability worked before, as it was the only sane way that I could do any sort of testing when I developed this pack.