This was working for me, but found a similar error on deleting a site.
I had to Network Deactivate the plugin to delete the site. After doing so, I got the error "Multisite Global Search requires multisite installation. Please create a network first, or deactivate Multisite Global Search." and was unable to reactivate the plugin.
Creating a new site allowed me to reactivate the plugin.
I can replicate this by deactivating a site (plugin breaks) and reactivating it (plugin works again.)
I'd be grateful for any ideas?
(On Version 1.2.11 of plugin, WordPress 3.5.2)