Fix #8037: assertion failed when reloading AI for last existing company in offline mode #8587
Motivation / Problem
Reloading AI means deleting and recreating a company.
First commit prevents deletion of last existing company.
Checklist for review
Some things are not automated, and forgotten often. This list is a reminder for the reviewers.
The mention of
Not sure what we should call non-multiplayer, but for sure we use different names for it currently :) Maybe good to tackle that for this PR already?