The issue is that if you do not set permissions in the manifest, clicking allow in the safari UI would only return a blank URI, "". The original extension in this case would also just return an error.
I've modified the manifest so that it is asking Safari for wider permissions, so that when the permission is granted by the user, the proper URL is returned.
Currently the extension will suggest that it needs access to every page the user visits, occasionally opening a popup automatically if I remember correctly.
For something that’s a glorified bookmarklet, that’s a lot to ask.