I wasn't able to reproduce this, since I haven't yet had the situation arise again... however, here's what happened:
1. At starfleet academy, I placed a Doff (a white conn officer I think) in a group to be reassigned by the Personnel Officer.
2. I then opened my Fleet Base interface and attempted to contribute that same officer to the base's project that was looking for white conn officers. The officer showed in the list.
3. I can't remember if I clicked Collect Rewards on the assignment interface or not at this point... but reassignment assignment completed, the conn officer vanished from the Fleet UI and my game client crashed out. I put this description in the crash feedback thing too but thought perhaps I should put it here too just in case.
Anyway, as I said, i haven't been able to replicate this because it's sort of an odd convergence of circumstances that I haven't been able to put myself in the position to do again (all my fleet's assignments are running, once we're on to the next ones, I can give it another go)