Attempting to delete a flight plan immediately crashes the game - Jordan release

[Jakathan]

Using the recent Jordan release, when attempting to delete a flight plan causes an immediate crash.

I have confirmed that this occurs in a clean vanilla 1.12.3 install with only Principia and the stock DLCs.

Recreation steps:

1. Create Flight Plan
2. Attempt to delete said flight plan

Link to the logs from both KSP and Principia after performing the above actions: Logs

Looks like the flight plan index somehow gets set to -1?