SUMMARY
If I have a laptop with an external screen connected and the laptop screen disabled, if I try to change the refresh rate of the disabled laptop screen, the system settings app will crash (see attached error log).

STEPS TO REPRODUCE
1. Have a laptop with an external screen enabled and the laptop screen disabled.
2. Go to system settings -> Display and Monitor -> Choose the laptop screen -> change the refresh rate.

OBSERVED RESULT
Crashes.

EXPECTED RESULT
Should not crash.

SOFTWARE/OS VERSIONS
Operating System: Debian GNU/Linux
KDE Plasma Version: 5.23.0
KDE Frameworks Version: 5.86.0
Qt Version: 5.15.2
Kernel Version: 5.14.0-3-amd64 (64-bit)
Graphics Platform: X11
Processors: 16 × AMD Ryzen 7 2700 Eight-Core Processor
Memory: 15.5 GiB of RAM
Graphics Processor: AMD VEGA10

ADDITIONAL INFORMATION
Application: System Settings (systemsettings5), signal: Segmentation fault

[KCrash Handler]
#4  0x00007f60c8dc5ed0 in KScreen::Mode::size() const () from /lib/x86_64-linux-gnu/libKF5Screen.so.7
#5  0x00007f60c8e23950 in ?? () from /usr/lib/x86_64-linux-gnu/qt5/plugins/kcms/kcm_kscreen.so
#6  0x00007f6137c08615 in ?? () from /lib/x86_64-linux-gnu/libQt5QmlModels.so.5
#7  0x00007f61393f5730 in QQmlPropertyPrivate::write(QObject*, QQmlPropertyData const&, QVariant const&, QQmlContextData*, QFlags<QQmlPropertyData::WriteFlag>) () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#8  0x00007f6139322a5b in QV4::QObjectWrapper::setProperty(QV4::ExecutionEngine*, QObject*, QQmlPropertyData*, QV4::Value const&) () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#9  0x00007f6139323457 in QV4::QObjectWrapper::setQmlProperty(QV4::ExecutionEngine*, QQmlContextData*, QObject*, QV4::String*, QV4::QObjectWrapper::RevisionMode, QV4::Value const&) () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#10 0x00007f61393235f5 in QV4::QObjectWrapper::virtualPut(QV4::Managed*, QV4::PropertyKey, QV4::Value const&, QV4::Value*) () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#11 0x00007f61392eea4b in QV4::Object::virtualResolveLookupSetter(QV4::Object*, QV4::ExecutionEngine*, QV4::Lookup*, QV4::Value const&) () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#12 0x00007f613933bec8 in ?? () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#13 0x00007f613933fcf7 in ?? () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#14 0x00007f61392db56d in QV4::Function::call(QV4::Value const*, QV4::Value const*, int, QV4::ExecutionContext const*) () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#15 0x00007f61394523c5 in QQmlJavaScriptExpression::evaluate(QV4::CallData*, bool*) () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#16 0x00007f613940658b in QQmlBoundSignalExpression::evaluate(void**) () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#17 0x00007f6139406a70 in ?? () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#18 0x00007f61394361ad in QQmlNotifier::emitNotify(QQmlNotifierEndpoint*, void**) () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#19 0x00007f613a5934dd in ?? () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#20 0x00007f612ff06b6e in QQuickComboBox::activated(int) () from /lib/x86_64-linux-gnu/libQt5QuickTemplates2.so.5
#21 0x00007f612fe90208 in ?? () from /lib/x86_64-linux-gnu/libQt5QuickTemplates2.so.5
#22 0x00007f613a593977 in ?? () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#23 0x00007f612fe7ff41 in QQuickAbstractButtonPrivate::handleRelease(QPointF const&) () from /lib/x86_64-linux-gnu/libQt5QuickTemplates2.so.5
#24 0x00007f612fe9cfe9 in QQuickControl::mouseReleaseEvent(QMouseEvent*) () from /lib/x86_64-linux-gnu/libQt5QuickTemplates2.so.5
#25 0x00007f613982d388 in QQuickItem::event(QEvent*) () from /lib/x86_64-linux-gnu/libQt5Quick.so.5
#26 0x00007f613b15174f in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#27 0x00007f613a55ce9a in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#28 0x00007f6139849314 in QQuickWindowPrivate::deliverMouseEvent(QQuickPointerMouseEvent*) () from /lib/x86_64-linux-gnu/libQt5Quick.so.5
#29 0x00007f613984a64b in QQuickWindowPrivate::deliverPointerEvent(QQuickPointerEvent*) () from /lib/x86_64-linux-gnu/libQt5Quick.so.5
#30 0x00007f613aa64825 in QWindow::event(QEvent*) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#31 0x00007f613b15174f in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#32 0x00007f613a55ce9a in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#33 0x00007f6139b69ea7 in QQuickWidget::mouseReleaseEvent(QMouseEvent*) () from /lib/x86_64-linux-gnu/libQt5QuickWidgets.so.5
#34 0x00007f613b1937ae in QWidget::event(QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#35 0x00007f613b15174f in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#36 0x00007f613b158d1b in QApplication::notify(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#37 0x00007f613a55ce9a in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#38 0x00007f613b157d53 in QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool, bool) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#39 0x00007f613b1acb9c in ?? () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#40 0x00007f613b1afe15 in ?? () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#41 0x00007f613b15174f in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Widgets.so.5
#42 0x00007f613a55ce9a in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#43 0x00007f613aa587a3 in QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent*) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#44 0x00007f613aa2e40c in QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Gui.so.5
#45 0x00007f61350bc4fa in ?? () from /lib/x86_64-linux-gnu/libQt5XcbQpa.so.5
#46 0x00007f6138081d0b in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#47 0x00007f6138081fb8 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#48 0x00007f613808206f in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#49 0x00007f613a5b4a90 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#50 0x00007f613a55b8db in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#51 0x00007f613a563b10 in QCoreApplication::exec() () from /lib/x86_64-linux-gnu/libQt5Core.so.5
#52 0x000056042cb3de30 in ?? ()
#53 0x00007f6139f04e4a in __libc_start_main (main=0x56042cb3d5f0, argc=1, argv=0x7ffc472ada08, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffc472ad9f8) at ../csu/libc-start.c:314
#54 0x000056042cb3eb5a in ?? ()
[Inferior 1 (process 3021) detached]