Found the little critter!!
OK, your observation was strange until you realise what was happening.
The sitecreationwizard is a whole load of javascript that is dragged into the platform on demand, unfortunately in the case of the widget, it was not loading it in prior to trying to call it.
A bit like trying to eat a pizza before you've actually ordered it.
So, if you log out then log in again, go to the plus circle, select site, wait for the site wizard to appear, then cancel it, then go to your button, it will work
That can be fixed ... but next week