Illegal string offset in Stylesheet pro-site error on themes page

After upgrading to WP 3.5, and upgrading all plugins, I get an error message when going to the themes directory:

Warning: Illegal string offset ‘Stylesheet’ in /home/writerwe/public_html/wp-content/plugins/pro-sites/pro-sites-files/modules/premium-themes.php on line 43

That error message is repeated for each theme I have installed (whether network active or not).

To troubleshoot, I disabled EVERY plugin except Pro-Sites. Error still showed. When I disabled Pro-Sites, error goes away. When I re-enable it, it’s back.

I looked through the sections but couldn’t find anything specific about this. Also, I copied over a new download of this plugin (although it was the same version).

Any help is greatly appreciated!