Fatal error on Adv. theme switcher plugin

I'm trying to install the Advanced Theme Switcher plugin to be able to create a demo site for all my themes. I am getting the following error:

Fatal error: Call to a member function get() on a non-object in /home/user/public_html/wp-includes/query.php on line 27

These are the plugins I have installed, and if they're in the mu-plugins folder or not:
Supporter (wpmu)
Maintenance Mode (wp)
Ozh Admin Drop Down Menu (wp)

Can anybody help? Thanks.