If you click the "Import now!" button on https://listenbrainz.org/profile/import while the Last.fm Username field is blank, it looks like it still attempts to start the import (which then fails with a JS exception because the last.fm api returns no data or an error). At this point the page is covered with a modal that cannot be dismissed; it has to be reloaded to recover.

It should instead present an error message stating that a Username is required.