Fatal Error happened when I start my packaged project

I have never had this problem (yet) but here is my guess about why it might be a problem. If the player BP is created first then it cannot get a reference to the animation BP which has not been created yet. But you cannot create the animation BP first because it would need a reference to a player BP which has not been created yet. But it seems to me that this is only a problem if the references are created at the time of BP construction. If both BP are constructed then afterwards they should be able to get a reference to each other. I could be completely wrong. But that would be my working theory.

I suppose what I would do is arrange the code so that the reference variables are only populated when everything has been constructed - maybe in beginplay. I would be interested to see the actual BP setup that you have. I like these sort of problems. They are a bit painful but you end up learning something. I like the programming challenge - I am not really very interested in the actual game