If I install debug symbols for libqt5webkit5 and I attach to the web process before triggering the crash, here is the backtrace I get:

#0  WebKit::WebPage::hidePopupMenu (this=0xae718a00) at WebProcess/WebPage/qt/WebPageQt.cpp:439
#1  0xb5de33ca in callMemberFunction<WebKit::WebPage, void (WebKit::WebPage::*)()> (function=<optimized out>, object=0xae718a00)
    at Platform/CoreIPC/HandleMessage.h:15
#2  handleMessage<Messages::WebPage::HidePopupMenu, WebKit::WebPage, void (WebKit::WebPage::*)()> (function=<optimized out>, 
    object=0xae718a00, decoder=...) at Platform/CoreIPC/HandleMessage.h:322
#3  WebKit::WebPage::didReceiveWebPageMessage (this=0xae718a00, decoder=...) at generated/WebPageMessageReceiver.cpp:476
#4  0xb5d976f6 in WebKit::WebPage::didReceiveMessage (this=0xae718a00, connection=0xae705dc0, messageID=..., decoder=...)
    at WebProcess/WebPage/WebPage.cpp:2922
#5  0xb5c5d365 in CoreIPC::MessageReceiverMap::dispatchMessage (this=0x9eeed74, connection=0xae705dc0, messageID=..., decoder=...)
    at Platform/CoreIPC/MessageReceiverMap.cpp:86
#6  0xb5da08d9 in WebKit::WebProcess::didReceiveMessage (this=0x9eeed40, connection=0xae705dc0, messageID=..., decoder=...)
    at WebProcess/WebProcess.cpp:681
#7  0xb5c5a440 in dispatchMessage (decoder=..., messageID=..., this=0xae705dc0) at Platform/CoreIPC/Connection.cpp:663
#8  CoreIPC::Connection::dispatchMessage (this=this@entry=0xae705dc0, message=...) at Platform/CoreIPC/Connection.cpp:686
#9  0xb5c5a559 in CoreIPC::Connection::dispatchOneMessage (this=0xae705dc0) at Platform/CoreIPC/Connection.cpp:712
#10 0xb5c5971f in operator() (c=<optimized out>, this=0xab03a380) at ../WTF/wtf/Functional.h:173
#11 WTF::BoundFunctionImpl<WTF::FunctionWrapper<void (CoreIPC::Connection::*)()>, void (CoreIPC::Connection*)>::operator()() (
    this=0xab03a378) at ../WTF/wtf/Functional.h:405
#12 0xb61e7106 in operator() (this=<synthetic pointer>) at ../WTF/wtf/Functional.h:613
#13 WebCore::RunLoop::performWork (this=0xae703780) at platform/RunLoop.cpp:87
#14 0xb62a3e16 in performWork (this=<optimized out>) at platform/qt/RunLoopQt.cpp:48
#15 qt_static_metacall (_id=0, _o=0x9eee610, _c=<optimized out>, _a=<optimized out>) at .moc/release-shared/RunLoopQt.moc:68
#16 WebCore::RunLoop::TimerObject::qt_static_metacall (_o=0x9eee610, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0xadd06710)
    at .moc/release-shared/RunLoopQt.moc:63
#17 0xb52917d3 in QMetaCallEvent::placeMetaCall(QObject*) () from /usr/lib/i386-linux-gnu/libQt5Core.so.5
#18 0xb52948db in QObject::event(QEvent*) () from /usr/lib/i386-linux-gnu/libQt5Core.so.5
#19 0xb556370c in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /usr/lib/i386-linux-gnu/libQt5Widgets.so.5
#20 0xb556713b in QApplication::notify(QObject*, QEvent*) () from /usr/lib/i386-linux-gnu/libQt5Widgets.so.5
#21 0xb526b74e in QCoreApplication::notifyInternal(QObject*, QEvent*) () from /usr/lib/i386-linux-gnu/libQt5Core.so.5
#22 0xb526d673 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () from /usr/lib/i386-linux-gnu/libQt5Core.so.5
#23 0xb526dd3c in QCoreApplication::sendPostedEvents(QObject*, int) () from /usr/lib/i386-linux-gnu/libQt5Core.so.5
#24 0xb52b9434 in ?? () from /usr/lib/i386-linux-gnu/libQt5Core.so.5
#25 0xb392f9e3 in g_main_context_dispatch () from /lib/i386-linux-gnu/libglib-2.0.so.0
#26 0xb392fd80 in ?? () from /lib/i386-linux-gnu/libglib-2.0.so.0
#27 0xb392fe61 in g_main_context_iteration () from /lib/i386-linux-gnu/libglib-2.0.so.0
#28 0xb52b95af in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
   from /usr/lib/i386-linux-gnu/libQt5Core.so.5
---Type <return> to continue, or q <return> to quit---
#29 0xafc77d36 in ?? () from /usr/lib/i386-linux-gnu/qt5/plugins/platforms/libqxcb.so
#30 0xb5269fd6 in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/i386-linux-gnu/libQt5Core.so.5
#31 0xb526a48c in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib/i386-linux-gnu/libQt5Core.so.5
#32 0xb526dde2 in QCoreApplication::exec() () from /usr/lib/i386-linux-gnu/libQt5Core.so.5
#33 0xb62a3e87 in WebCore::RunLoop::run () at platform/qt/RunLoopQt.cpp:69
#34 0xb5da8545 in WebKit::WebProcessMainQt (app=0x9e79058) at WebProcess/qt/WebProcessMainQt.cpp:195
#35 0x080488b8 in main (argc=2, argv=0xbfa2cb34) at qt/MainQt.cpp:100