It appears that an invalid type was added, resulting in an empty type. The plugin definitely needs a sanity check for input validation.
For now, you can use the Rails console to delete the empty type:
cd /var/discourse
./launcher enter app
rails c
DiscourseRatings::RatingType.destroy("")
exit
Refresh the page, and it should be gone!
Keep the type simple, such as my_type, and you are good. 