So looking into this further the ruler view is apparently accessing my custom view instance (the content view of the scroller view the ruler belongs to) after the view hierarchy has been teared down.
This is a new bug in the 10.12.2 beta.
Some behaviour in NSRulerView must have changed - why would it try to access the view (it doesn't own/have a strong reference to) after the entire view hierarchy was detached? Pity, it's causing constant crashes in my app as users can toggle views on/off to switch from a 2D to a 2.5D view.
Apple, please fix this before releasing 10.12.2 to the public.
I was having a similar problem and thought it was a bug in the beta too but I think it's just a change in behavior that exposes a bug. I could trigger the crash pre beta using methods related to the ruler view that I know should be safe so there was definitely something going on with my code. Check to see if you used setClientView:aView on your ruler view and if so, setClientView:nil before releasing aView. That solved my problem.
Awesome - after stripping down my app to the base minimum this turned out to be the issue as well for me.
Still believe this is a bug that we now manually have to reset the content view.
Anywho, all clear now for a public 10.12.2!