Bug 247302

EventDispatcher has a null ScrollingTree: Thread 16 Crashed:: Dispatch queue: com.apple.WebKit.EventDispatcher 0 WebCore 0x71bff0bd9 bool std::__1::__cxx_atomic_compare_exchange_weak<unsigned char>(std::__1::__cxx_atomic_base_impl<unsigned char>*, unsigned char*, unsigned char, std::__1::memory_order, std::__1::memory_order) + 2 (atomic:1050) [inlined] 1 WebCore 0x71bff0bd9 std::__1::__atomic_base<unsigned char, false>::compare_exchange_weak(unsigned char&, unsigned char, std::__1::memory_order) + 2 (atomic:1681) [inlined] 2 WebCore 0x71bff0bd9 WTF::Atomic<unsigned char>::compareExchangeWeak(unsigned char, unsigned char, std::__1::memory_order) + 2 (Atomics.h:89) [inlined] 3 WebCore 0x71bff0bd9 WTF::LockAlgorithm<unsigned char, (unsigned char)1, (unsigned char)2, WTF::EmptyLockHooks<unsigned char> >::lockFastAssumingZero(WTF::Atomic<unsigned char>&) + 2 (LockAlgorithm.h:53) [inlined] 4 WebCore 0x71bff0bd9 WTF::Lock::lock() + 2 (Lock.h:65) [inlined] 5 WebCore 0x71bff0bd9 WTF::Locker<WTF::Lock>::Locker(WTF::Lock&) + 2 (Lock.h:158) [inlined] 6 WebCore 0x71bff0bd9 WTF::Locker<WTF::Lock>::Locker(WTF::Lock&) + 2 (Lock.h:157) [inlined] 7 WebCore 0x71bff0bd9 WebCore::ScrollingTree::hasProcessedWheelEventsRecently() + 25 (ScrollingTree.cpp:784) 8 WebCore 0x71bffad05 WebCore::ThreadedScrollingTree::scrollingThreadIsActive() + 5 (ThreadedScrollingTree.cpp:354) [inlined] 9 WebCore 0x71bffad05 WebCore::ThreadedScrollingTree::displayDidRefresh(unsigned int) + 21 (ThreadedScrollingTree.cpp:532) 10 WebKit 0x71330b04b WebKit::EventDispatcher::notifyScrollingTreesDisplayWasRefreshed(unsigned int) + 181 (EventDispatcher.cpp:306) 11 WebKit 0x71330b0eb WebKit::EventDispatcher::displayWasRefreshed(unsigned int, WebCore::DisplayUpdate const&, bool) + 67 (EventDispatcher.cpp:321)