How do I track down an unspecified System.NullReferenceException?

That only shows up when I’m running on the device (iOS), and not in the editor?
I’m getting an occasional (the worst kind) crash, upon returning to the game’s main menu, and in the xCode output there’s this…

**Unhandled Exception: System.NullReferenceException:** 

…and that’s all she wrote. If I could get it to happen when I’m previewing it in Unity it would tell me what caused the error, or
if xCode barfed up its usual screenful of calls with the offending line highlighted, I could get this fixed, but right now it’s a mystery.
Before the error in the output, I’ve got this:

Unloading 1 unused Assets to reduce memory usage. Loaded Objects now: 29976. Operation took 850.822998 ms.
GameCenter logged in

Running LoadPrefsForOptionsScreen
 
(Filename: /Applications/buildAgent/work/14194e8ce88cdf47/Runtime/ExportGenerated/iPhonePlayer-armv7/UnityEngineDebug.cpp Line: 43)

2012-09-29 09:50:10.738 fourdsnake[1549:907] no Facebook session available

From that I’m guessing it is something with my prefs and connecting to Facebook, but the Facebook stuff is Prime31’s and my prefs handling is kind of a mess so that doesn’t help.

Can I force xCode to give me more detailed error messages in this case?

I was able to track down my issue by changing Script Call Optimization from ‘Fast but no exceptions’ to ‘Slow and safe’ in the players settings.