The issue:

Installed the extension in Chrome (83.0.4103.106 (Official Build) (64-bit))
Clicked the extension.
Clicked "Import account"
Clicked "Select"
Clicked "Choose file"
The "open file" dialog popped up, but the extension popup closed / disappeared, so when I select a file, there's nowhere for it to go.
Clicking the extension again, starts over instead of remembering where it left off.

The suggested fix:

Suggestion if the extension popup can't be force to stay open: provide an "Options" page which can do the setup. ("Options" can be seen on all extensions in the right-click menu, but many have it disabled, including this one.)

Thanks to Ed Hennis for identifying this issue.