Sorry, I cannot reproduce this. Please install debug symbols for the package that contains QtWidgets and then create a new backtrace that does not contain any "??" values anymore. Otherwise it is not possible to see where the crash originates from.
On some distributions the required package is called "qtbase5-dbg".

After installing debug symbols, I get the below report:

#0  QtSharedPointer::ExternalRefCountData::getAndRef (obj=obj@entry=0x17d4780) at tools/qsharedpointer.cpp:1343
        d = 0xb25070
        x = <optimized out>
#1  0x00007ffff3ae2d98 in QWeakPointer<QObject>::QWeakPointer<QObject> (ptr=0x17d4780, this=0x7fffffffcf60)
    at ../../include/QtCore/../../src/corelib/tools/qsharedpointer_impl.h:696
No locals.
#2  QWeakPointer<QObject>::assign<QObject> (ptr=0x17d4780, this=0x7ffff3fd1b00 <qt_last_mouse_receiver>)
    at ../../include/QtCore/../../src/corelib/tools/qsharedpointer_impl.h:692
No locals.
#3  QPointer<QWidget>::operator= (p=0x17d4780, this=0x7ffff3fd1b00 <qt_last_mouse_receiver>) at ../../include/QtCore/../../src/corelib/kernel/qpointer.h:67
No locals.
#4  QWidgetWindow::handleMouseEvent (this=this@entry=0x2587270, event=event@entry=0x7fffffffd360) at kernel/qwidgetwindow.cpp:453
        receiver = 0x17d4780
        widgetPos = <optimized out>
        alien = 0x0
        e = {<QInputEvent> = {<QEvent> = {_vptr.QEvent = 0x7ffff3910a58 <vtable for QMouseEvent+16>, static staticMetaObject = {d = {superdata = 0x0, 
                  stringdata = 0x7ffff2b83440 <qt_meta_stringdata_QEvent>, data = 0x7ffff2b82ea0 <qt_meta_data_QEvent>, static_metacall = 0x0, relatedMetaObjects = 0x0, 
                  extradata = 0x0}}, d = 0x0, t = 3, posted = 0, spont = 0, m_accept = 1, reserved = 75}, modState = {i = 0}, ts = 606500}, l = {xp = 93, yp = 42}, w = {
            xp = 93, yp = 42}, s = {xp = 141, yp = 114}, b = Qt::LeftButton, mouseState = {i = 0}, caps = 0, velocity = {xp = 0, yp = 0}}
        popupChild = 0x0
        activePopupWidget = 0x17d4780
        popup = 0x17d4780
        mapped = {xp = 93, yp = 42}
        releaseAfter = true
        oldOpenPopupCount = 1
        contextMenuTrigger = QEvent::MouseButtonPress
        widget = <optimized out>
        mapped = {xp = -11424, yp = 32767}
        receiver = <optimized out>
#5  0x00007ffff3ae521b in QWidgetWindow::event (this=0x2587270, event=0x7fffffffd360) at kernel/qwidgetwindow.cpp:210
No locals.
#6  0x00007ffff3a81f95 in QApplicationPrivate::notify_helper (this=this@entry=0x7de3c0, receiver=receiver@entry=0x2587270, e=e@entry=0x7fffffffd360)
    at kernel/qapplication.cpp:3716
        consumed = <optimized out>
#7  0x00007ffff3a870ba in QApplication::notify (this=0x7fffffffd790, receiver=0x2587270, e=0x7fffffffd360) at kernel/qapplication.cpp:3160
        res = false
#8  0x00007ffff29ae0ab in QCoreApplication::notifyInternal (this=0x7fffffffd790, receiver=receiver@entry=0x2587270, event=event@entry=0x7fffffffd360)
    at kernel/qcoreapplication.cpp:965
        result = false
        cbdata = {0x2587270, 0x7fffffffd360, 0x7fffffffd22f}
        d = <optimized out>
        threadData = 0x7d3870
        loopLevelCounter = {threadData = 0x7d3870}
#9  0x00007ffff34b83dd in QCoreApplication::sendSpontaneousEvent (event=0x7fffffffd360, receiver=0x2587270)
    at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:227
No locals.
#10 QGuiApplicationPrivate::processMouseEvent (e=0xa03ea0) at kernel/qguiapplication.cpp:1789
        type = QEvent::MouseButtonRelease
        stateChange = <optimized out>
        window = 0x2587270
        localPoint = {xp = 93, yp = 42}
        globalPoint = {xp = 141, yp = 114}
        button = Qt::LeftButton
        doubleClick = false
        frameStrut = false
        ev = {<QInputEvent> = {<QEvent> = {_vptr.QEvent = 0x7ffff3910a58 <vtable for QMouseEvent+16>, static staticMetaObject = {d = {superdata = 0x0, 
                  stringdata = 0x7ffff2b83440 <qt_meta_stringdata_QEvent>, data = 0x7ffff2b82ea0 <qt_meta_data_QEvent>, static_metacall = 0x0, relatedMetaObjects = 0x0, 
                  extradata = 0x0}}, d = 0x0, t = 3, posted = 0, spont = 1, m_accept = 1, reserved = 0}, modState = {i = 0}, ts = 606500}, l = {xp = 93, yp = 42}, w = {
            xp = 93, yp = 42}, s = {xp = 141, yp = 114}, b = Qt::LeftButton, mouseState = {i = 0}, caps = 0, velocity = {xp = 0, yp = 0}}
#11 0x00007ffff34ba125 in QGuiApplicationPrivate::processWindowSystemEvent (e=e@entry=0xa03ea0) at kernel/qguiapplication.cpp:1581
No locals.
#12 0x00007ffff349c910 in QWindowSystemInterface::sendWindowSystemEvents (flags=...) at kernel/qwindowsysteminterface.cpp:625
        event = 0xa03ea0
        nevents = 1
#13 0x00007fffe4294ddb in userEventSourceDispatch (source=<optimized out>) at eventdispatchers/qeventdispatcher_glib.cpp:70
        userEventSource = <optimized out>
        dispatcher = <optimized out>
#14 0x00007fffed805dc7 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#15 0x00007fffed806020 in ?? () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#16 0x00007fffed8060cc in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
No symbol table info available.
#17 0x00007ffff2a0775e in QEventDispatcherGlib::processEvents (this=0x827120, flags=...) at kernel/qeventdispatcher_glib.cpp:418
        d = 0x825bf0
        canWait = true
        savedFlags = {i = 0}
        result = <optimized out>
#18 0x00007fffe4294fb5 in QPAEventDispatcherGlib::processEvents (this=<optimized out>, flags=...) at eventdispatchers/qeventdispatcher_glib.cpp:115
No locals.
#19 0x00007ffff29ab65a in QEventLoop::exec (this=this@entry=0x7fffffffd6f0, flags=..., flags@entry=...) at kernel/qeventloop.cpp:204
        d = 0x18061f0
        locker = {val = 8206720}
        ref = {d = 0x18061f0, locker = @0x7fffffffd670, exceptionCaught = true}
#20 0x00007ffff29b3b7b in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1229
        threadData = 0x7d3870
        eventLoop = {<QObject> = {_vptr.QObject = 0x7ffff2bfb3d8 <vtable for QEventLoop+16>, static staticMetaObject = {d = {superdata = 0x0, 
                stringdata = 0x7ffff2b25d80 <qt_meta_stringdata_QObject>, data = 0x7ffff2b25c60 <qt_meta_data_QObject>, 
                static_metacall = 0x7ffff29e6a00 <QObject::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}, 
            d_ptr = {d = 0x18061f0}, static staticQtMetaObject = {d = {superdata = 0x0, stringdata = 0x7ffff2b6dac0 <qt_meta_stringdata_Qt>, 
                data = 0x7ffff2b6b0c0 <qt_meta_data_Qt>, static_metacall = 0x0, relatedMetaObjects = 0x0, extradata = 0x0}}}, static staticMetaObject = {d = {
              superdata = 0x7ffff2bf2a40 <QObject::staticMetaObject>, stringdata = 0x7ffff2b82a80 <qt_meta_stringdata_QEventLoop>, 
              data = 0x7ffff2b82a20 <qt_meta_data_QEventLoop>, 
              static_metacall = 0x7ffff2a61f50 <QEventLoop::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)>, relatedMetaObjects = 0x0, extradata = 0x0}}}
        returnCode = <optimized out>
#21 0x00007ffff34af70b in QGuiApplication::exec () at kernel/qguiapplication.cpp:1527
No locals.
#22 0x00007ffff3a7d81a in QApplication::exec () at kernel/qapplication.cpp:2976
No locals.
#23 0x0000000000436334 in main ()
No symbol table info available.

Trying to reproduce it in a bunch of subsequent releases as well as development snapshots after opening this bug report, I can no longer face the issue. I consider the issue as fixed, so I think that I can close it.