具体报错如下，希望可以得到解决：
2014-06-28 08:57:46.508 XYZ[707:1403] *** Assertion failure in -[UIKeyboardTaskQueue waitUntilAllTasksAreFinished], /SourceCache/UIKit_Sim/UIKit-2935.137/Keyboard/UIKeyboardTaskQueue.m:368
2014-06-28 08:57:46.509 XYZ[707:1403] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[UIKeyboardTaskQueue waitUntilAllTasksAreFinished] may only be called from the main thread.'
*** First throw call stack:
(
0 CoreFoundation 0x0266e1e4 __exceptionPReprocess + 180
1 libobjc.A.dylib 0x023ed8e5 objc_exception_throw + 44
2 CoreFoundation 0x0266e048 +[NSException raise:format:arguments:] + 136
3 Foundation 0x01fcd4de -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 116
4 UIKit 0x01800166 -[UIKeyboardTaskQueue waitUntilAllTasksAreFinished] + 167
5 UIKit 0x0128ff11 -[UIKeyboardImpl setDelegate:force:] + 442
6 UIKit 0x0128fd52 -[UIKeyboardImpl setDelegate:] + 60
7 UIKit 0x01556dc5 -[UIPeripheralHost(UIKitInternal) _reloadInputViewsForResponder:] + 956
8 UIKit 0x01560a0e -[UIPeripheralHost(UIKitInternal) _preserveInputViewsWithId:animated:] + 550
9 UIKit 0x011d7940 -[UIViewController presentViewController:withTransition:completion:] + 3317
10 UIKit 0x10f9078f -[UIViewControlleraccessibility(SafeCategory) presentViewController:withTransition:completion:] + 71
11 UIKit 0x011d8aef -[UIViewController presentViewController:animated:completion:] + 130
12 XYZ 0x0007e613 __33-[LoginViewController UserLogin:]_block_invoke + 483
13 Foundation 0x02108ac5 __67+[NSURLConnection sendAsynchronousRequest:queue:completionHandler:]_block_invoke_2 + 151
14 Foundation 0x02068d65 -[NSBlockOperation main] + 88
15 Foundation 0x020c1c79 -[__NSOperationInternal _start:] + 671
16 Foundation 0x0203e9c8 -[NSOperation start] + 83
17 Foundation 0x020c3f44 __NSOQSchedule_f + 62
18 libdispatch.dylib 0x030854d0 _dispatch_client_callout + 14
19 libdispatch.dylib 0x03071fe0 _dispatch_async_redirect_invoke + 202
20 libdispatch.dylib 0x030854d0 _dispatch_client_callout + 14
21 libdispatch.dylib 0x03073eb7 _dispatch_root_queue_drain + 291
22 libdispatch.dylib 0x03074127 _dispatch_worker_thread2 + 39
23 libsystem_pthread.dylib 0x033b4dab _pthread_wqthread + 336
24 libsystem_pthread.dylib 0x033b8cce start_wqthread + 30
)
libc++abi.dylib: terminating with uncaught exception of type NSException