Reminders App Hangs after Mavericks Upgrade A couple of people on twitter share this issue, but I cannot find any discussions of it nor solutions. 

Since moving to OS X 10.9, Reminders app keeps pinwheeling, and won't respond. It doesn't crash, so I have to force-quit it. This happens on both my 2008 iMac and my 2013 Air. On my Air, I can tell something weird is happening because upon launching Reminders the fans kick on loud. 

The data seems to be syncing fine from iCloud, so it doesn't look like a sync issue is causing this.

Any ideas about what could be causing this issue and how to fix it?
I started to do the same as mentioned above -removing all location-based reminders including completed ones... (had to go directly to iCloud.com)

As I was going through, I found one that had "location: null", deleted it (or just removed the location part of it) and then, before deleting any more, tried my reminder app on my Mac computer (running Mavericks that wouldn't open/crashed). AND IT WORKED

So to be clear, you don't have to delete ALL location based reminders -just any that happen to have "null" in the location which, of course, messes with it's head! (I must've bumped it on/not done it right).