Installing an extension from vsix raises a "No Servers" exception

I've been trying to make my extension ([betterfountain](/piersdeseilligny/betterfountain)) compatible with github1s (installing it currently causes the error "Cannot install 'Better Fountain' because this extension has defined that it cannot run on the web server").

In order to do so, I want to install any test versions through a vsix file, rather than publishing it to the marketplace simply for a test.

Currently, however, it appears impossible, given that selecting "Install from vsix" and then picking a vsix file in the currently open repo, raises an extremely vague "No Servers" error, with no more details than that in any log:


I've also tested with a .vsix of the vim extension, which definitely works without any issues, and it's the same error. Presumably this is a bug? Ideally it would be nice to be able to upload vsix files, rather than them needing to be on the repo, but I realize that's probably quite a bit more complicated.