Using 4.18 and the Fish map, I see the crash happen. I can look into it
Edit:
Solved: those fish are children of “FlockFish”, which is managed by BP_FishManager, using them directly results in the crash because they have no reference to BP_FishManager!
Solution : Drag n drop BP_FishManager into scene, and in the right hand details pane, you can add “Flock Types” which is just an array of BP actors that use the FlockFish as a parent
You can even customize the number of fish!