I think you’re not supposed to use non-serializable types as BBParameter variables. This happens because NC tries to create a NavMeshPath instance when it loads the graph and unity doesn’t allow that (similarly to how you can’t access some other APIs during certain points like serialization or construction). I guess you’re trying to have one action generate the path where you can store it and then re-use it in some other action to apply it. But I’m unfortunately not having an idea how this could be solved properly. I guess something in the underlying serialization logic must be changed so it does not try to persist NavMeshPath instances anywhere.
