MPPTaskScheduleEntry::waitForSchedule null pointer caused tiflash crash

[yibin87]

[BaseDaemon.cpp:563] [\"\
       0x7715391\\tfaultSignalHandler(int, siginfo_t*, void*) [tiflash+124867473]\
                \\tlibs/libdaemon/src/BaseDaemon.cpp:214\
  0x7f514bc6ddf0\\t<unknown symbol> [libc.so.6+347632]\
       0x887d390\\tDB::MPPTaskScheduleEntry::waitForSchedule() [tiflash+143119248]\
                \\tdbms/src/Flash/Mpp/MPPTaskScheduleEntry.cpp:66\
       0x88640a4\\tDB::MPPTask::runImpl() [tiflash+143016100]\
                \\tdbms/src/Flash/Mpp/MPPTask.cpp:538\
       0x1fb48d8\\tauto DB::wrapInvocable<std::__1::function<void ()> >(bool, std::__1::function<void ()>&&)::'lambda'()::operator()() [tiflash+33245400]\ 
                \\tdbms/src/Common/wrapInvocable.h:36\
       0x1ea7243\\tDB::DynamicThreadPool::executeTask(std::__1::unique_ptr<DB::IExecutableTask, std::__1::default_delete<DB::IExecutableTask> >&) [tiflash+32141891]\
                \\tdbms/src/Common/DynamicThreadPool.cpp:124\
       0x1ea6bd1\\tDB::DynamicThreadPool::fixedWork(unsigned long) [tiflash+32140241]\
                \\tdbms/src/Common/DynamicThreadPool.cpp:138\
       0x1ea88b2\\tvoid* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, std::__1::thread DB::ThreadFactory::newThread<void (DB::DynamicThreadPool::*)(unsigned long), DB::DynamicThreadPool*, unsigned long&>(bool, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, void (DB::DynamicThreadPool::*&&)(unsigned long), DB::DynamicThreadPool*&&, unsigned long&)::'lambda'(auto&&...), DB::DynamicThreadPool*, unsigned long> >(void*) [tiflash+32147634]\
                \\t/usr/local/bin/../include/c++/v1/thread:291\
  0x7f514bcb8802\\tstart_thread [libc.so.6+653314]\"] [source=BaseDaemon] [thread_id=1072]