FWeakObjectPtr::IsValid() cause crash!

I am using UE4 with hairork branch to do a multiplayer game. But some time the app crash with this error from log message:
[2015.10.26-08.55.45:994][834]LogWindows: === Critical error: ===
Fatal error!

UnderwaterViz.exe!FWeakObjectPtr::IsValid() (0x000000013ff26a2c) + 145 bytes [e:\unrealengine-hairworks\engine\source\runtime\coreuobject\private\uobject\weakobjectptr.cpp:243]
UnderwaterViz.exe!FParallelAnimationEvaluationTask::DoTask() (0x00000001414f1213) + 5 bytes [e:\unrealengine-hairworks\engine\source\runtime\engine\private\skeletalmeshcomponent.cpp:56]
UnderwaterViz.exe!TGraphTask::ExecuteTask() (0x00000001414fbd86) + 0 bytes [e:\unrealengine-hairworks\engine\source\runtime\core\public\async\taskgraphinterfaces.h:671]
UnderwaterViz.exe!FTaskThread::ProcessTasks() (0x000000013fb1bc2f) + 0 bytes [e:\unrealengine-hairworks\engine\source\runtime\core\private\async\taskgraph.cpp:428]
UnderwaterViz.exe!FTaskThread::ProcessTasksUntilQuit() (0x000000013fb1bdfd) + 0 bytes [e:\unrealengine-hairworks\engine\source\runtime\core\private\async\taskgraph.cpp:271]
UnderwaterViz.exe!FTaskThread::Run() (0x000000013fb2781b) + 0 bytes [e:\unrealengine-hairworks\engine\source\runtime\core\private\async\taskgraph.cpp:562]
UnderwaterViz.exe!FRunnableThreadWin::Run() (0x000000013fda3296) + 0 bytes [e:\unrealengine-hairworks\engine\source\runtime\core\private\windows\windowsrunnablethread.cpp:73]
UnderwaterViz.exe!FRunnableThreadWin::GuardedRun() (0x000000013fd9b39d) + 8 bytes [e:\unrealengine-hairworks\engine\source\runtime\core\private\windows\windowsrunnablethread.cpp:26]
kernel32.dll!UnknownFunction (0x000000007749652d) + 0 bytes [UnknownFile:0]
ntdll.dll!UnknownFunction (0x00000000775cc521) + 0 bytes [UnknownFile:0]
ntdll.dll!UnknownFunction (0x00000000775cc521) + 0 bytes [UnknownFile:0]

Crash in runnable thread TaskGraphThread 1

[2015.10.26-08.55.46:114][834]LogExit: Executing StaticShutdownAfterError
[2015.10.26-08.55.46:114][834]LogWindows:Error: Error reentered: Runnable thread TaskGraphThread 1 crashed.
[2015.10.26-08.55.46:114][834]LogWindows:Error: HandleError re-entered.
[2015.10.26-08.55.46:114][834]LogWindows: FPlatformMisc::RequestExit(1)
[2015.10.26-08.55.46:114][834]Log file closed, 10/26/15 16:55:4

Can you please help me foudn out which can cause these error?

You don’t have any inforamtion above this log?

Sorry, here is the whole log file