AI opponent with Meddling Mage breaks server! (The reason of Beta AI disabled)

If an AI player puts a [[Meddling Mage]] on the battlefield, the game doesn't allow you to press the "Submit" button during the sideboarding. A window appears with this error. You also can't start a new game before the server restart.


Also, third Meddling Mage on the battlefield didn't have a selected card

Start by reproducing an AI sideboarding game with Meddling Mage and inspect the displayed server error. Trace the handling of Meddling Mage choices and sideboarding submission, including the case of a third Mage without a selected card. Done means Submit works, a new game can start without a server restart, and every Mage has a selection.

Written by the indexing model from the issue text.