Question Unity crashes after adding an avatar to the Animator.

I finished all my animations and blend trees for this character, and wanted to move on to making a preliminary "Head Aim" using the Animation Rigging package. Everything worked fine until I pressed play, then the editor crashed.

I have deduced that it is something about the avatar causing the crash. When I use the aim rig without the avatar in the Animator component reference, it works perfectly. As soon as I add the avatar it crashes when I press play.

I am using the character model and bones from the most recent third person character controller by unity at the moment.
I have searched reddit, google and submitted a bug report too.

Thank you for your help!