title: Crash with preferences (NSUserDefaults)?
body: <p>I'm having this crash now and then, in a thread that is not started by me that I know, with CFXPreferencesPropertyListSourceSynchronizer and CFXPreferencesPropertyListSource mentioned.
  I do have a separate thread that accesses the NSUserDefaults.  (NSUserDefaults is supposed to be thread safe.) Anyone recognize what is going wrong here?</p>
  
  <pre><code>Exception Type:  EXC_BAD_ACCESS (SIGSEGV)
  Exception Codes: KERN_INVALID_ADDRESS at 0x444e4128
  Crashed Thread:  5
  
  Thread 5 name:  Dispatch queue: CFPreferences Background Sync Queue
  Thread 5 Crashed:
  0   libobjc.A.dylib                 0x34ec4f78 objc_msgSend + 16
  1   CoreFoundation                  0x3710de90 CFRetain + 76
  2   CoreFoundation                  0x3718aa96 __CFDictionaryStandardRetainValue + 62
  3   CoreFoundation                  0x37114fe0 CFBasicHashCreateCopy + 832
  4   CoreFoundation                  0x37124f50 CFDictionaryCreateMutableCopy + 388
  5   CoreFoundation                  0x3712d340 -[CFXPreferencesPropertyListSourceSynchronizer initWithPropertyListSource:forLockedSynchronize:] + 232
  6   CoreFoundation                  0x371b4800 __79-[CFXPreferencesPropertyListSource synchronizeInBackgroundWithCompletionBlock:]_block_invoke_0 + 56
  7   libdispatch.dylib               0x302e5c52 _dispatch_call_block_and_release + 6
  8   libdispatch.dylib               0x302f0ca4 _dispatch_queue_drain + 268
  9   libdispatch.dylib               0x302f0b12 _dispatch_queue_invoke$VARIANT$up + 30
  10  libdispatch.dylib               0x302f1784 _dispatch_worker_thread2 + 208
  11  libsystem_c.dylib               0x33a34df4 _pthread_wqthread + 288
  12  libsystem_c.dylib               0x33a34cc8 start_wqthread + 0
  </code></pre>