Opening QtCurve's settings crashes System Settings if the global menu is in use.

Backtrace:

Application: System Settings (systemsettings5), signal: Segmentation fault
Using host libthread_db library "/usr/lib/libthread_db.so.1".
[Current thread is 1 (Thread 0x7f30a41a57c0 (LWP 30187))]

Thread 3 (Thread 0x7f308367a700 (LWP 30190)):
#0  0x00007f309fe7367d in poll () from /usr/lib/libc.so.6
#1  0x00007f309826f7a6 in ?? () from /usr/lib/libglib-2.0.so.0
#2  0x00007f309826f8bc in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#3  0x00007f30a079b24b in QEventDispatcherGlib::processEvents (this=0x7f307c0008c0, flags=...) at kernel/qeventdispatcher_glib.cpp:425
#4  0x00007f30a0744b5a in QEventLoop::exec (this=this@entry=0x7f3083679dc0, flags=..., flags@entry=...) at kernel/qeventloop.cpp:212
#5  0x00007f30a0567a93 in QThread::exec (this=this@entry=0x7f30a1325d60 <(anonymous namespace)::Q_QGS__q_manager::innerFunction()::holder>) at thread/qthread.cpp:507
#6  0x00007f30a10b1835 in QDBusConnectionManager::run (this=0x7f30a1325d60 <(anonymous namespace)::Q_QGS__q_manager::innerFunction()::holder>) at qdbusconnection.cpp:170
#7  0x00007f30a056c6b8 in QThreadPrivate::start (arg=0x7f30a1325d60 <(anonymous namespace)::Q_QGS__q_manager::innerFunction()::holder>) at thread/qthread_unix.cpp:368
#8  0x00007f309beab2e7 in start_thread () from /usr/lib/libpthread.so.0
#9  0x00007f309fe7d54f in clone () from /usr/lib/libc.so.6

Thread 2 (Thread 0x7f30912d0700 (LWP 30189)):
#0  0x00007f309fe7367d in poll () from /usr/lib/libc.so.6
#1  0x00007f309c72f8e0 in ?? () from /usr/lib/libxcb.so.1
#2  0x00007f309c731679 in xcb_wait_for_event () from /usr/lib/libxcb.so.1
#3  0x00007f3093e669e9 in QXcbEventReader::run (this=0xe25020) at qxcbconnection.cpp:1345
#4  0x00007f30a056c6b8 in QThreadPrivate::start (arg=0xe25020) at thread/qthread_unix.cpp:368
#5  0x00007f309beab2e7 in start_thread () from /usr/lib/libpthread.so.0
#6  0x00007f309fe7d54f in clone () from /usr/lib/libc.so.6

Thread 1 (Thread 0x7f30a41a57c0 (LWP 30187)):
[KCrash Handler]
#6  0x00007f30a1b47474 in QFlags<Qt::WindowType>::operator& (f=<optimized out>, this=<optimized out>) at ../../include/QtCore/../../src/corelib/global/qflags.h:142
#7  QWindow::type (this=this@entry=0x166fbd0) at kernel/qwindow.cpp:826
#8  0x00007f30a1b4cac2 in QWindow::winId (this=0x166fbd0) at kernel/qwindow.cpp:599
#9  0x00007f308bdbe312 in ?? () from /usr/lib/qt/plugins/platformthemes/KDEPlasmaPlatformTheme.so
#10 0x00007f308bdbe9ed in ?? () from /usr/lib/qt/plugins/platformthemes/KDEPlasmaPlatformTheme.so
#11 0x00007f30a2458a3c in QMenuBarPrivate::handleReparent (this=this@entry=0x166f3f0) at widgets/qmenubar.cpp:1362
#12 0x00007f30a2459715 in QMenuBar::eventFilter (this=0x166f0a0, object=0x1642320, event=0x7ffe407ce7f0) at widgets/qmenubar.cpp:1462
#13 0x00007f30a0746461 in QCoreApplicationPrivate::sendThroughObjectEventFilters (receiver=receiver@entry=0x1642320, event=event@entry=0x7ffe407ce7f0) at kernel/qcoreapplication.cpp:1099
#14 0x00007f30a22dc385 in QApplicationPrivate::notify_helper (this=<optimized out>, receiver=0x1642320, e=0x7ffe407ce7f0) at kernel/qapplication.cpp:3741
#15 0x00007f30a22e3bc1 in QApplication::notify (this=0x7ffe407d0970, receiver=0x1642320, e=0x7ffe407ce7f0) at kernel/qapplication.cpp:3502
#16 0x00007f30a0746700 in QCoreApplication::notifyInternal2 (receiver=receiver@entry=0x1642320, event=event@entry=0x7ffe407ce7f0) at kernel/qcoreapplication.cpp:988
#17 0x00007f30a2324d66 in QCoreApplication::sendEvent (event=0x7ffe407ce7f0, receiver=0x1642320) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:231
#18 QWidget::setParent (this=this@entry=0x1642320, parent=parent@entry=0x16980d0, f=...) at kernel/qwidget.cpp:10597
#19 0x00007f30a2325afc in QWidget::setParent (this=this@entry=0x1642320, parent=parent@entry=0x16980d0) at kernel/qwidget.cpp:10451
#20 0x00007f30a22ff8f6 in QLayout::addChildWidget (this=this@entry=0x1689150, w=w@entry=0x1642320) at kernel/qlayout.cpp:926
#21 0x00007f30a22ff96f in QLayout::addWidget (this=0x1689150, w=0x1642320) at kernel/qlayout.cpp:226
#22 0x00007f30a243f43c in QMdiSubWindow::setWidget (this=this@entry=0x16980d0, widget=widget@entry=0x1642320) at widgets/qmdisubwindow.cpp:2330
#23 0x00007f30a24335a3 in QMdiArea::addSubWindow (this=<optimized out>, widget=0x1642320, windowFlags=...) at widgets/qmdiarea.cpp:1989
#24 0x00007f307908e2dd in QtCurveConfig::previewControlPressed() () from /usr/lib/qt/plugins/kstyle_qtcurve5_config.so
#25 0x00007f307908e3f9 in QtCurveConfig::setupPreview() () from /usr/lib/qt/plugins/kstyle_qtcurve5_config.so
#26 0x00007f3079099ab5 in QtCurveConfig::QtCurveConfig(QWidget*) () from /usr/lib/qt/plugins/kstyle_qtcurve5_config.so
#27 0x00007f3079099b51 in allocate_kstyle_config () from /usr/lib/qt/plugins/kstyle_qtcurve5_config.so
#28 0x00007f3079316daf in ?? () from /usr/lib/qt/plugins/kcm_style.so
#29 0x00007f30a07722de in QtPrivate::QSlotObjectBase::call (a=0x7ffe407cfba0, r=0x12cbbf0, this=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qobject_impl.h:101
#30 QMetaObject::activate (sender=sender@entry=0x12d3a90, signalOffset=<optimized out>, local_signal_index=local_signal_index@entry=2, argv=argv@entry=0x7ffe407cfba0) at kernel/qobject.cpp:3723
#31 0x00007f30a0772887 in QMetaObject::activate (sender=sender@entry=0x12d3a90, m=m@entry=0x7f30a29b73e0 <QAbstractButton::staticMetaObject>, local_signal_index=local_signal_index@entry=2, argv=argv@entry=0x7ffe407cfba0) at kernel/qobject.cpp:3602
#32 0x00007f30a23c4b92 in QAbstractButton::clicked (this=this@entry=0x12d3a90, _t1=<optimized out>) at .moc/moc_qabstractbutton.cpp:309
#33 0x00007f30a23c4de4 in QAbstractButtonPrivate::emitClicked (this=0x12d3bc0) at widgets/qabstractbutton.cpp:411
#34 0x00007f30a23c61c7 in QAbstractButtonPrivate::click (this=0x12d3bc0) at widgets/qabstractbutton.cpp:404
#35 0x00007f30a23c63a4 in QAbstractButton::mouseReleaseEvent (this=0x12d3a90, e=0x7ffe407d0080) at widgets/qabstractbutton.cpp:1009
#36 0x00007f30a2323c18 in QWidget::event (this=0x12d3a90, event=0x7ffe407d0080) at kernel/qwidget.cpp:9156
#37 0x00007f30a22dc3ac in QApplicationPrivate::notify_helper (this=<optimized out>, receiver=0x12d3a90, e=0x7ffe407d0080) at kernel/qapplication.cpp:3745
#38 0x00007f30a22e4925 in QApplication::notify (this=<optimized out>, receiver=0x12d3a90, e=0x7ffe407d0080) at kernel/qapplication.cpp:3219
#39 0x00007f30a0746700 in QCoreApplication::notifyInternal2 (receiver=receiver@entry=0x12d3a90, event=event@entry=0x7ffe407d0080) at kernel/qcoreapplication.cpp:988
#40 0x00007f30a22e312d in QCoreApplication::sendEvent (event=<optimized out>, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:231
#41 QApplicationPrivate::sendMouseEvent (receiver=receiver@entry=0x12d3a90, event=event@entry=0x7ffe407d0080, alienWidget=alienWidget@entry=0x12d3a90, nativeWidget=0xf23e20, buttonDown=buttonDown@entry=0x7f30a29e1810 <qt_button_down>, lastMouseReceiver=..., spontaneous=true) at kernel/qapplication.cpp:2715
#42 0x00007f30a233e316 in QWidgetWindow::handleMouseEvent (this=this@entry=0x1058780, event=event@entry=0x7ffe407d04a0) at kernel/qwidgetwindow.cpp:617
#43 0x00007f30a2340d33 in QWidgetWindow::event (this=0x1058780, event=0x7ffe407d04a0) at kernel/qwidgetwindow.cpp:239
#44 0x00007f30a22dc3ac in QApplicationPrivate::notify_helper (this=<optimized out>, receiver=0x1058780, e=0x7ffe407d04a0) at kernel/qapplication.cpp:3745
#45 0x00007f30a22e3bc1 in QApplication::notify (this=0x7ffe407d0970, receiver=0x1058780, e=0x7ffe407d04a0) at kernel/qapplication.cpp:3502
#46 0x00007f30a0746700 in QCoreApplication::notifyInternal2 (receiver=receiver@entry=0x1058780, event=event@entry=0x7ffe407d04a0) at kernel/qcoreapplication.cpp:988
#47 0x00007f30a1b42b13 in QCoreApplication::sendSpontaneousEvent (event=0x7ffe407d04a0, receiver=0x1058780) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:234
#48 QGuiApplicationPrivate::processMouseEvent (e=0x12f1930) at kernel/qguiapplication.cpp:1927
#49 0x00007f30a1b44685 in QGuiApplicationPrivate::processWindowSystemEvent (e=e@entry=0x12f1930) at kernel/qguiapplication.cpp:1711
#50 0x00007f30a1b201eb in QWindowSystemInterface::sendWindowSystemEvents (flags=...) at kernel/qwindowsysteminterface.cpp:710
#51 0x00007f3093ec7010 in userEventSourceDispatch (source=<optimized out>) at qeventdispatcher_glib.cpp:76
#52 0x00007f309826f5a7 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#53 0x00007f309826f810 in ?? () from /usr/lib/libglib-2.0.so.0
#54 0x00007f309826f8bc in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#55 0x00007f30a079b22f in QEventDispatcherGlib::processEvents (this=0xe608d0, flags=...) at kernel/qeventdispatcher_glib.cpp:423
#56 0x00007f30a0744b5a in QEventLoop::exec (this=this@entry=0x7ffe407d0850, flags=..., flags@entry=...) at kernel/qeventloop.cpp:212
#57 0x00007f30a074d094 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1261
#58 0x000000000040c2b4 in ?? ()
#59 0x00007f309fdb1511 in __libc_start_main () from /usr/lib/libc.so.6
#60 0x000000000040c33a in _start ()