Same issue as OP, including same error message. Now holding back from upgrading your plugin because I'm unwilling to lose my existing settings.
Before finding this thread, I attempted to upgrade the plugin this morning without upgrading to WP 3.5.2. Got the error message reported by OP and 500 error.
Deleted your plugin, site returned and ran normally. Uploaded the old version of your plugin from this morning's backup, and site still worked fine.
Upgraded to WP 3.5.2, tested site with no problems. Your plugin's former version ran fine through the WP upgrade and after. Again attempted to upgrade your plugin; site again went 500, returning same error in line 271. Deleted plugin to get site back (effective) and then again uploaded my last backup's version of the plugin instead.
Site runs fine. Honestly, I'm happy with the way the plugin works. I've also got many, many page-specific configurations of plugins using your plugin, so if the only fix is to delete it completely and do a fresh install, I'm not willing to do that just to get your new version at this point.
If there is a way I can do a fresh install of your plugin from the WP repositories yet retain all my old settings, including existing post/page-specific configurations, great. If not, I didn't think the plugin was broken before and will stick with your last iteration of V.2.