I've seen this happen before. This particular time it's on a button controller which is a REAL pain cause I will have to delete the whole button controller and re-create it. (Deleting the individual button action didnt fix it)
Is there any reason why this happens?
Did you happen to use the browser's "Back" button when creating this rule (or otherwise click away from the page using anything other than the buttons in Rule Machine/Button Controller)? That's one way you can get a rule/BC to think it's "stuck" in a certain UI state, and it's generally advised against. I'm not aware of any way to fix this, unless it's stuck at forcing you to add an action (yours isn't), in which case you can just go through with that and delete it later.
I might have. If that is often what does it, I will need to be really careful.
It's probabably the #1 cause.