This is 100% reproduceable. Try it out. Maybe instead of crashing the game, the game should provide a warning that it will not allow a sov to imbue with it's last 3 essence.
This is late in the mid game of a large map and my Sov had 17 mana so it was no evident to me immediately that it was my last three but I have three shards and several Temples of Essence.
SD Please add this to your bugs to be fixed list along with the Kingdom/Mace/warhammer Tech tree.
I understand that the Mana pool system should prevent this from happening but the game should not crash to desktop it should prevent the illegal action gracefully.