WWW IOS Crash

Unity version : 5.5.0f3 Game is crashed randomly. Why it is happen ?

 Crashed: com.unity3d.WebOperationQueue :: NSOperation 0x170059470 (QOS: DEFAULT)
 EXC_BAD_ACCESS KERN_INVALID_ADDRESS 0x0000000000000000
 Crashed: com.unity3d.WebOperationQueue :: NSOperation 0x170059470 (QOS: DEFAULT)
 0  space                          0x100b78f88 WWW::FeedUnityWebStream(bool) (WWW.h:96)
 1  space                          0x100e826c0 iPhoneWWWDownloadBuffer::OnCompleteContent() (iPhoneWWW.mm:64)
 2  space                          0x100e226f4 UnityReportWWWFinishedLoadingData (TransportiPhone.cpp:260)
 3  space                          0x100069f04 -[UnityWWWConnectionDelegate connectionDidFinishLoading:] (WWWConnection.mm:246)
 4  CFNetwork                      0x18db912cc __65-[NSURLConnectionInternal _withConnectionAndDelegate:onlyActive:]_block_invoke + 80
 5  CFNetwork                      0x18db9125c -[NSURLConnectionInternal _withConnectionAndDelegate:onlyActive:] + 200
 6  CFNetwork                      0x18db913d0 -[NSURLConnectionInternal _withActiveConnectionAndDelegate:] + 56
 7  CFNetwork                      0x18dab35cc ___ZN27URLConnectionClient_Classic26_delegate_didFinishLoadingEU13block_pointerFvvE_block_invoke + 96
 8  CFNetwork                      0x18dab14bc ___ZN27URLConnectionClient_Classic18_withDelegateAsyncEPKcU13block_pointerFvP16_CFURLConnectionPK33CFURLConnectionClientCurrent_VMaxE_block_invoke_2 + 108
 9  libdispatch.dylib              0x18c1be1bc _dispatch_client_callout + 16
 10 libdispatch.dylib              0x18c1c9ab0 _dispatch_block_invoke_direct + 376
 11 CFNetwork                      0x18db6e2a8 RunloopBlockContext::_invoke_block(void const*, void*) + 36
 12 CoreFoundation                 0x18d20dc18 CFArrayApplyFunction + 68
 13 CFNetwork                      0x18db6e18c RunloopBlockContext::perform() + 136
 14 CFNetwork                      0x18db6f4b4 MultiplexerSource::perform() + 312
 15 CFNetwork                      0x18db6f220 MultiplexerSource::_perform(void*) + 64
 16 CoreFoundation                 0x18d2e2b5c __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 24
 17 CoreFoundation                 0x18d2e24a4 __CFRunLoopDoSources0 + 524
 18 CoreFoundation                 0x18d2e00a4 __CFRunLoopRun + 804
 19 CoreFoundation                 0x18d20e2b8 CFRunLoopRunSpecific + 444
 20 Foundation                     0x18dd4b26c -[NSRunLoop(NSRunLoop) runMode:beforeDate:] + 304
 21 Foundation                     0x18dd9faa0 -[NSRunLoop(NSRunLoop) run] + 88
 22 space                          0x10006adf8 __UnitySendWWWConnection_block_invoke_2 (WWWConnection.mm:362)
 23 Foundation                     0x18de23fb0 __NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ + 16
 24 Foundation                     0x18dd68aa8 -[NSBlockOperation main] + 96
 25 Foundation                     0x18dd590a4 -[__NSOperationInternal _start:] + 620
 26 Foundation                     0x18de2635c __NSOQSchedule_f + 228
 27 libdispatch.dylib              0x18c1be1bc _dispatch_client_callout + 16
 28 libdispatch.dylib              0x18c1cc3dc _dispatch_queue_serial_drain + 928
 29 libdispatch.dylib              0x18c1c19a4 _dispatch_queue_invoke + 652
 30 libdispatch.dylib              0x18c1ce34c _dispatch_root_queue_drain + 572
 31 libdispatch.dylib              0x18c1ce0ac _dispatch_worker_thread3 + 124
 32 libsystem_pthread.dylib        0x18c3c72a0 _pthread_wqthread + 1288
 33 libsystem_pthread.dylib        0x18c3c6d8c start_wqthread + 4