I think it's because your sovereign is either smart or meditates. Kingdoms no longer want those "philosophical" sovereigns leading their nations. AWAY WITH THEE!

No, it's a bug. They're fixing it... and hopefully soon!

Thank you for reporting this.  We have identified the source of the crash.

You will get the crash if you give your sovereign one of the following abilities: brilliant, meditative, and/or merchant.

We are testing a build with the fix internally, and will be will be releasing a hotfix as soon as possible.

In the meantime, if you don't use any of those three abilities, you won't get the crash.