First time posting so I'm not sure what kind of details you are looking for, but I found this to be a very persistent bug.
Whenever I am trying to upgrade an army of let's say more than 10 cultists into liches WHILE they are in combat it crashes the game consistently.
I found that if I scrolls away from the battle and upgrade nothing goes wrong and the fight goes on, but if I look at them growing up the screen starts blowing up (Ha, I'm so clever)
Thought I would post it here to let you guys know.
Large groups of cultists in combat that are upgraded to liches cause a crash.