I had the same error with my template.

If u look for "sj_plus" in templates folder, you can see it's missing.

I don't know if it's the best solution but i've changed this folder name and added the missing underscore.

You also have to add this underscore in the database. You'll find your template name in a table called "extensions".

Now it works for me, but that's only my personnal view

Perhaps SmartAddons team will have a better workaround.