Creating GUI for extensions: "No valid expandable route found for {}"

Been going through the docs for creating an API with API v2. I can see some parts are out of date, and have been noting them and am happy to give some pointers from someone looking at it fresh but I'm sure you probably know where.

However I think there's a part that's more critical. In the final section of the docs when adding a GUI, it throws up an error when launching the eV: "No valid expandable route found for timelapse".

I've tracked this down to build_gui_from_dict(gui_description, extension_object) in gui.py. It seems to be an issue with cleaning the leading slashes as it's trying to match form["route"] (= timelapse) with extension_object._rules.keys() (= /timelapse). If I comment lines 20 and 21 out of gui.py (i.e. don't remove the leading slashes) I can use the extension (but seems to break other routes).