CheckboxButtonGroup active list broken in 0.12.14

Hi there,

I opened issue 7562 about two weeks ago, which is about the broken CheckboxButtonGroups in 0.12.14: Clicking buttons deselects certain other buttons in the group. Due to this bug, which affects the official example website, too, I cannot update to 0.12.14.

As far as I can see, the issue has not gained any attention on GitHub.

It feels like this bug is just a typo in a for-loop or something… maybe it has to do with the switch to TypeScript?

I looked at this commit and the coffeescript version, but I don’t have enough knowledge to see if there is a harmful difference. I would enjoy contributing by solving this issue, but I have no idea how. :-/