Version 0.20 was released today. Maybe that will fix your issue.
Edit: Here are the release notes.
Subler 0.20
Updated TheMovieDB api.
Added iTunes Store metadata importer.
Improved ratings selection.
Srt -> tx3g conversion improvement: font color, subtitles position (bottom, top), forced subtitles. (refer to the SrtFileFormat wiki page for an example).
Various bug fixed.