very minor bug alert 
- Go into Admin
- Click Groups
- Click New
- Change your mind. You don’t really want a new group. Click Delete.
- The following pops up. Clicking “Yes” does nothing at all.

very minor bug alert 

The problem might not be that minor. The routes after clicking new is http://localhost:3000/admin/groups/custom/undefined which isn’t right.
I’ve submitted a PR to fix this. https://github.com/discourse/discourse/pull/3782