Save editor + save.
But I suggest trying to remove the boat summon yourself, just in case, you never know =)
The bug is certainly not a pleasant one. I previously tried summoning a boat via the shipyard onto an active attack event; the trick there is that we can summon a boat this way through the shipyard, but not through a regular ship summon, and after boarding the boat, we don't activate the event even though we are standing on it—but it activates after we sail away. That is, it works on the same principle as Dimension Door onto an event; those who played Last Crusade might know what I mean. But the fact that AI attempts to summon a boat via magic lead to a crash is, of course, a significant issue.
Upd. Updated the link.
Thank you very much! It works!
I suggest taking a look at the map, by the way. It's made in the best traditions: micro-battles during the 1st week with neutrals so as not to lose the army, and then—an invasion of enemy heroes.