I placed down a ballista in creative mode and spawned a siege engineer. He mounted it and started aiming at a nearby zombie. The firing animation went off and then the game crashed. The engineer firing off a loaded ballista seems to work fine tho.
Here is the crash report:
https://pastebin.com/7nMYXP7s

Yeah I found that one shortly after the release, will likely do a fix tomorrow. For now what you can do to prevent that is put some ammo in the vehicle. Engineer won't use it, but it will stop this crash from happening.

Released a fix for this yesterday, closing.