最近，有关__NSOperationInternal的崩溃很多。详细信息如下：
Thread 2 name:
Thread 2 Crashed:
0 Foundation 0x0000000185848b88 oq_has_observer + 112 (NSOperation.m:0)
1 Foundation 0x0000000185788680 +[__NSOperationInternal _observeValueForKeyPath:ofObject:changeKind:oldValue:newValue:indexes:context:] + 1776 (NSOperation.m:2219)
2 Foundation 0x0000000185787f58 -[__NSOperationInternal _start:] + 1532 (NSOperation.m:0)
3 Foundation 0x000000018584876c __NSOQSchedule_f + 404 (NSOperation.m:2081)
4 libdispatch.dylib 0x0000000184710a60 _dispatch_client_callout + 16 (object.m:507)
5 libdispatch.dylib 0x0000000184718e94 _dispatch_continuation_pop$VARIANT$mp + 424 (inline_internal.h:2500)
6 libdispatch.dylib 0x00000001847177cc _dispatch_async_redirect_invoke$VARIANT$mp + 604 (queue.c:3422)
7 libdispatch.dylib 0x000000018471dcac _dispatch_root_queue_drain + 588 (inline_internal.h:2539)
8 libdispatch.dylib 0x000000018471d9fc _dispatch_worker_thread3 + 120 (queue.c:6092)
9 libsystem_pthread.dylib 0x0000000184a43fac _pthread_wqthread + 1176 (pthread.c:2297)
10 libsystem_pthread.dylib 0x0000000184a43b08 start_wqthread + 4