What steps will reproduce the problem?
1. Build & run
2. Complete OAuth login to Twitter
3. Crash

See:

https://groups.google.com/forum/#!topic/gtm-oauth/N6jlOpL9k5g

This line of code causes the crash:

- (void)destroyWindow {
  // no request; close the window (but not immediately, in case
  // we're called in response to some window event)
  [[self webView] stopLoading:nil]; <--- *** causes the crash
  . . .

Here's a stack trace of the crash:

2014-04-17 14:55:44.000 -0- Exception handler: Uncaught system exception: 
signal 11
2014-04-17 14:55:44.084 -0- Stack Trace:
#0  ExceptionHandling                   000140735561141361 
NSExceptionHandlerUncaughtSignalHandler()
#1  libsystem_platform.dylib            000140735578928554 _sigtramp()
#2  ??                                  000000004660728840 
+[SyncableObjectMO_SyncableObject_ _PFPlaceHolderSingleton]
#3  WebCore                             000140735485213608 
_ZN7WebCore14DocumentLoader14maybeLoadEmptyEv()
#4  WebCore                             000140735485213103 
_ZN7WebCore14DocumentLoader24startLoadingMainResourceEv()
#5  WebCore                             000140735485834990 
_ZN7WebCore11FrameLoader33continueLoadAfterNavigationPolicyERKNS_15ResourceReque
stEN3WTF10PassRefPtrINS_9FormStateEEEb()
#6  WebCore                             000140735485834064 
_ZN7WebCore11FrameLoader37callContinueLoadAfterNavigationPolicyEPvRKNS_15Resourc
eRequestEN3WTF10PassRefPtrINS_9FormStateEEEb()
#7  WebCore                             000140735485833877 
_ZN7WebCore14PolicyCallback4callEb()
#8  WebCore                             000140735485833760 
_ZN7WebCore13PolicyChecker29continueAfterNavigationPolicyENS_12PolicyActionE()
#9  WebKit                              000140735616624699 
-[WebFramePolicyListener receivedPolicyDecision:]
#10 CoreFoundation                      000140735597009900 __invoking___()
#11 CoreFoundation                      000140735597009492 -[NSInvocation 
invoke]
#12 CoreFoundation                      000140735597676006 -[NSInvocation 
invokeWithTarget:]
#13 CoreFoundation                      000140735597478340 ___forwarding___()
#14 CoreFoundation                      000140735597477752 
_CF_forwarding_prep_0()
#15 WebKit                              000140735616622308 
_ZN20WebFrameLoaderClient39dispatchDecidePolicyForNavigationActionEMN7WebCore13P
olicyCheckerEFvNS0_12PolicyActionEERKNS0_16NavigationActionERKNS0_15ResourceRequ
estEN3WTF10PassRefPtrINS0_9FormStateEEE()
. . .

To fix the crash, comment out the -stopLoading line.