There is a bug on the Button element.
When I try to add a class on the Style-> Id&Class → Button (tab) the first time after writing an pressing enter (or the + sign), the actual class is not added and the Button (tab) gets renamed to (Undefined).
After I’m doing this again the class is added but the Button (tab) remains (Undefined).
The same issue is when I try to remove the class that I’ve added, clicking on the Trash icon the first time is not working.
Here is a video with the issue.