This is a known issue, and is related to limitations regarding how Unity draws UnityEvents. It's a hard one to fix, and it looks like we're essentially just going to have to make our very own UnityEvent drawer, which I'm afraid is a little bit in the future. For now, you'll have to make sure that your UnityEvents are serialized by Unity, and are in a Component. The next patch does have an improved error message that essentially states this, though. Sorry about this.