So I was able to replicate this bug several times in different instances.
Steps to replicate crash:
Have an army with low cohesion and begin a siege
The army will break apart, leaving the player character still sieging
Finish siege prep (any sort of siege equipment will work) and lead the assult
Game crash
This also happens when an lord is sieging a castle by himself and you decide to help by also laying siege in your on separate camp and the lord who is in charge of the siege leaves. The player takes command of the siege prep and when you select lead the assault, game crashes.
Steps to replicate crash:
Have an army with low cohesion and begin a siege
The army will break apart, leaving the player character still sieging
Finish siege prep (any sort of siege equipment will work) and lead the assult
Game crash
This also happens when an lord is sieging a castle by himself and you decide to help by also laying siege in your on separate camp and the lord who is in charge of the siege leaves. The player takes command of the siege prep and when you select lead the assault, game crashes.

