Hi Paleta,
Thank you for providing the project. I was able to see the crash that you described, and when I associated it with our latest internal build I was able to get the Engine to give me a callstack with more information in it. This appears to be an issue with a circular dependency. I started looking at the other Blueprints that were included in the project and discovered that I would get the same crash if I tried to open the I_CharacterPickup Blueprint Interface. If I removed one of these Blueprints from the project, I could open the other one successfully (though the interface did give some additional trouble when trying to open it without the MyCharacter Blueprint in the project).
Interfaces are usually helpful in avoiding circular references, but I am not sure it was completely successful in this instance. I did some digging to see if I could find out exactly where things were going wrong, but unfortunately I was not able to narrow it down much further. I think the Item Bag Blueprint may be involved as well. If you could, take a close look at those three Blueprints to make sure you do not have a circular dependency between them.
Our Blueprint team is working on reducing the impact of circular dependencies in Blueprints. Some improvements have already been released, but there is still a lot of work to do.