The only way to get Fleet ID null is to try to spawn a fleet with a null id. It might sound like I'm being condescending, but I'm not. We're just establishing the logic by which you'll be able to debug this.
Next you'll need to post the source code for all your spawners. If you only want to post the pertinent bits, here's what I'm really looking for:
- Any line where you call spawn fleet.
- Any line which assigns a value to a variable which is passed as any of the arguments to the above.
That will tell us if it's ever possible to have a null or not. I'm hoping that you've simply overlooked something, but we'll get to the bottom of it.