WordPress 4.1 Support

it works on my 4.1

Yeah, I just tested again and it seems to be working.
Maybe my JSON was invalid and it seems your example JSON code is also invalid. It might have been just that.

Actually no, as soon as I tried to change anything (I just replaced the word “Buttons” with “Columns” and that broke it again.

Screenshot

try the json code first on: https://jsonlint.com/
as is wrote in guide

If your JSON is still invalid, you can use the new JSON-Generator (version 1.4).

Awesome, thanks!