Error when double-clicking unmounted location

[jtojnar]

Thanks for adding the double-click support, I was just about to ask for it. I noticed one issue, though. When I double click an unmounted drive, it raises the following error:

Traceback (most recent call last):
  File "/nix/store/njsi0vygq275igv5ns2qsjp3hxavbc5r-sunflower-2020-07-16-unstable/lib/python3.8/site-packages/sunflower/widgets/location_menu.py", line 138, in __handle_location_activate
    self.__handle_open_click(widget)
  File "/nix/store/njsi0vygq275igv5ns2qsjp3hxavbc5r-sunflower-2020-07-16-unstable/lib/python3.8/site-packages/sunflower/widgets/location_menu.py", line 154, in __handle_open_click
    self._active_object.change_path(selected_location.get_location())
  File "/nix/store/njsi0vygq275igv5ns2qsjp3hxavbc5r-sunflower-2020-07-16-unstable/lib/python3.8/site-packages/sunflower/plugins/file_list/file_list.py", line 2277, in change_path
    if (self.scheme is None or self.scheme == 'file') and '://' in path:
TypeError: argument of type 'NoneType' is not iterable

Ideally, it would be mounted and then opened.