Also reproducible in KDE Neon testing edition.

Thread 1 (Thread 0x7f35a92b46c0 (LWP 5549)):
[KCrash Handler]
#6  0x00007f358807a954 in TouchpadConfigContainer::sizeHint (this=<optimized out>) at ./kcms/touchpad/src/kcm/touchpadconfigcontainer.cpp:70
#7  0x00007f35ae8817c2 in QWidgetItemV2::updateCacheIfNecessary (this=0x55cdb8dcacf0) at kernel/qlayoutitem.cpp:724
#8  QWidgetItemV2::updateCacheIfNecessary (this=0x55cdb8dcacf0) at kernel/qlayoutitem.cpp:719
#9  0x00007f35ae881afd in QWidgetItemV2::maximumSize (this=0x55cdb8dcacf0) at kernel/qlayoutitem.cpp:812
#10 QWidgetItemV2::maximumSize (this=0x55cdb8dcacf0) at kernel/qlayoutitem.cpp:806
#11 0x00007f35ae8760a2 in QBoxLayoutPrivate::setupGeom (this=this@entry=0x55cdb9628170) at kernel/qboxlayout.cpp:289
#12 0x00007f35ae876d48 in QBoxLayoutPrivate::setupGeom (this=0x55cdb9628170) at kernel/qboxlayout.cpp:256
#13 QBoxLayout::sizeHint (this=<optimized out>) at kernel/qboxlayout.cpp:616
#14 0x00007f35ae87d8d7 in QLayout::totalSizeHint (this=0x55cdb9701120) at kernel/qlayout.cpp:719
#15 0x00007f35ae9eabaa in QScrollArea::sizeHint (this=<optimized out>) at /usr/include/c++/8/bits/atomic_base.h:390
#16 0x00007f35ae8835b2 in QStackedLayout::sizeHint (this=<optimized out>) at kernel/qstackedlayout.cpp:444
#17 0x00007f35ae87d8d7 in QLayout::totalSizeHint (this=0x55cdb7c25a40) at kernel/qlayout.cpp:719
#18 0x00007f35ae8817c2 in QWidgetItemV2::updateCacheIfNecessary (this=0x55cdb7c286f0) at kernel/qlayoutitem.cpp:724
#19 QWidgetItemV2::updateCacheIfNecessary (this=0x55cdb7c286f0) at kernel/qlayoutitem.cpp:719
#20 0x00007f35ae881aad in QWidgetItemV2::minimumSize (this=0x55cdb7c286f0) at kernel/qlayoutitem.cpp:799
#21 QWidgetItemV2::minimumSize (this=0x55cdb7c286f0) at kernel/qlayoutitem.cpp:793
#22 0x00007f35ae87a20d in QGridBox::minimumSize (this=0x55cdb7c28490) at kernel/qgridlayout.cpp:74
#23 QGridLayoutPrivate::setupLayoutData (this=this@entry=0x55cdb7c25500, hSpacing=hSpacing@entry=6, vSpacing=vSpacing@entry=6) at kernel/qgridlayout.cpp:810
#24 0x00007f35ae87b523 in QGridLayoutPrivate::distribute (this=this@entry=0x55cdb7c25500, r=..., hSpacing=6, vSpacing=vSpacing@entry=6) at kernel/qgridlayout.cpp:940
#25 0x00007f35ae87c322 in QGridLayout::setGeometry (this=0x55cdb7c254b0, rect=...) at kernel/qgridlayout.cpp:1385
#26 0x00007f35ae87d738 in QLayoutPrivate::doResize (this=this@entry=0x55cdb7c25500, r=...) at ../../include/QtCore/../../src/corelib/tools/qrect.h:184
#27 0x00007f35ae87e7c9 in QLayout::activate (this=<optimized out>) at ../../include/QtCore/../../src/corelib/tools/qsize.h:119
#28 0x00007f35ae89f61f in QWidgetPrivate::setVisible (this=0x55cdb8ef55a0, visible=<optimized out>) at ../../include/QtWidgets/../../src/widgets/kernel/qwidget.h:131
#29 0x00007f35ae883bb6 in QStackedLayout::setCurrentIndex (this=0x55cdb7c25a40, index=1) at kernel/qstackedlayout.cpp:321
#30 0x00007f35ae883e58 in QStackedLayout::setCurrentWidget (this=0x55cdb7c25a40, widget=widget@entry=0x55cdb96c96e0) at kernel/qstackedlayout.cpp:376
#31 0x00007f35aea007c5 in QStackedWidget::setCurrentWidget (this=<optimized out>, widget=0x55cdb96c96e0) at widgets/qstackedwidget.cpp:248
#32 0x00007f35af0b9125 in KPageViewPrivate::_k_pageSelected (previous=..., index=..., this=0x55cdb7c25270) at ./src/kpageview.cpp:247
#33 KPageViewPrivate::_k_pageSelected (this=0x55cdb7c25270, index=..., previous=...) at ./src/kpageview.cpp:221
#34 0x00007f35ade2a426 in QMetaObject::activate (sender=0x55cdb7c36b50, signalOffset=<optimized out>, local_signal_index=<optimized out>, argv=<optimized out>) at kernel/qobject.cpp:3793
#35 0x00007f35addc0108 in QItemSelectionModel::selectionChanged (this=this@entry=0x55cdb7c36b50, _t1=..., _t2=...) at .moc/moc_qitemselectionmodel.cpp:458
#36 0x00007f35addc89b4 in QItemSelectionModel::emitSelectionChanged (this=0x55cdb7c36b50, newSelection=..., oldSelection=...) at itemmodels/qitemselectionmodel.cpp:1871
#37 0x00007f35addc901b in QItemSelectionModel::select (this=0x55cdb7c36b50, selection=..., command=...) at itemmodels/qitemselectionmodel.cpp:1338
#38 0x00007f35addc2eac in QItemSelectionModel::select (this=this@entry=0x55cdb7c36b50, index=..., command=command@entry=...) at itemmodels/qitemselectionmodel.cpp:1188
#39 0x00007f35addc04dc in QItemSelectionModel::setCurrentIndex (this=this@entry=0x55cdb7c36b50, index=..., command=command@entry=...) at itemmodels/qitemselectionmodel.cpp:1413
#40 0x00007f35af0b81e7 in KPageViewPrivate::updateSelection (this=<optimized out>) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qabstractitemmodel.h:60
#41 KPageViewPrivate::updateSelection (this=<optimized out>) at ./src/kpageview.cpp:95
#42 0x00007f35af0b8f52 in KPageViewPrivate::_k_modelChanged (this=0x55cdb7c25270) at ./src/kpageview.cpp:218
#43 0x00007f35ade2a426 in QMetaObject::activate (sender=0x55cdb7c28f00, signalOffset=<optimized out>, local_signal_index=<optimized out>, argv=<optimized out>) at kernel/qobject.cpp:3793
#44 0x00007f35addb29e6 in QAbstractItemModel::layoutChanged (this=this@entry=0x55cdb7c28f00, _t1=..., _t2=<optimized out>, _t2@entry=QAbstractItemModel::NoLayoutChangeHint) at .moc/moc_qabstractitemmodel.cpp:570
#45 0x00007f35af0bf19b in KPageWidgetModel::addPage (this=0x55cdb7c28f00, item=item@entry=0x55cdb965df50) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qlist.h:151
#46 0x00007f35af0bdd9d in KPageWidget::addPage (this=<optimized out>, item=item@entry=0x55cdb965df50) at ./src/kpagewidget_p.h:35
#47 0x00007f35af3ca53c in ModuleView::addModule (this=this@entry=0x55cdb7c24e20, module=0x55cdb7c1d018) at ./core/ModuleView.cpp:198
#48 0x00007f35af3ca943 in ModuleView::loadModule (this=this@entry=0x55cdb7c24e20, menuItem=...) at ./core/ModuleView.cpp:151
#49 0x00007f3595e9d9e1 in SidebarMode::setActiveSubCategory (this=this@entry=0x55cdb7c21300, cat=4) at /usr/include/x86_64-linux-gnu/qt5/QtCore/qabstractitemmodel.h:60
#50 0x00007f3595ea34a7 in SidebarMode::qt_static_metacall (_o=_o@entry=0x55cdb7c21300, _c=_c@entry=QMetaObject::WriteProperty, _id=_id@entry=4, _a=_a@entry=0x7ffebe755880) at ./obj-x86_64-linux-gnu/sidebar/systemsettings_sidebar_mode_autogen/EWIEGA46WW/moc_SidebarMode.cpp:313
#51 0x00007f3595ea38f3 in SidebarMode::qt_metacall (this=0x55cdb7c21300, _c=QMetaObject::WriteProperty, _id=4, _a=0x7ffebe755880) at ./obj-x86_64-linux-gnu/sidebar/systemsettings_sidebar_mode_autogen/EWIEGA46WW/moc_SidebarMode.cpp:361
#52 0x00007f35ace17362 in QV4::QObjectWrapper::setProperty (engine=0x55cdb7c3faa0, object=0x55cdb7c21300, property=0x55cdb7d780e8, value=...) at ../../include/QtQml/5.12.2/QtQml/private/../../../../../src/qml/qml/qqmlpropertycache_p.h:214
#53 0x00007f35ace1807e in QV4::QObjectWrapper::setQmlProperty (value=..., revisionMode=QV4::QObjectWrapper::IgnoreRevision, name=<optimized out>, object=0x55cdb7c21300, qmlContext=<optimized out>, engine=0x55cdb7c3faa0) at jsruntime/qv4qobjectwrapper.cpp:435
#54 QV4::QObjectWrapper::setQmlProperty (engine=0x55cdb7c3faa0, qmlContext=<optimized out>, object=0x55cdb7c21300, name=<optimized out>, revisionMode=QV4::QObjectWrapper::IgnoreRevision, value=...) at jsruntime/qv4qobjectwrapper.cpp:418
#55 0x00007f35ace181cf in QV4::QObjectWrapper::virtualPut (m=0x7f358bbc1588, id=..., value=..., receiver=0x7f358bbc1588) at /usr/include/c++/8/bits/atomic_base.h:390
#56 0x00007f35ace964b7 in QV4::Object::put (receiver=<optimized out>, v=..., name=0x7f358bbc1580, this=0x7f358bbc1588) at ../../include/QtQml/5.12.2/QtQml/private/../../../../../src/qml/jsruntime/qv4string_p.h:171
#57 QV4::Runtime::method_storeProperty (engine=0x55cdb7c3faa0, object=..., nameIndex=<optimized out>, value=...) at jsruntime/qv4runtime.cpp:617
#58 0x00007f35ace2fe70 in QV4::Moth::VME::interpret (frame=0x7ffebe755bd0, engine=0x55cdb7c3faa0, code=0x7f35940292ee "\016\002`") at jsruntime/qv4vme_moth.cpp:605
#59 0x00007f35ace34297 in QV4::Moth::VME::exec (frame=frame@entry=0x7ffebe755bd0, engine=engine@entry=0x55cdb7c3faa0) at jsruntime/qv4vme_moth.cpp:441
#60 0x00007f35acdc55eb in QV4::Function::call (this=this@entry=0x55cdb7e43a60, thisObject=<optimized out>, argv=argv@entry=0x7f358bbc1500, argc=<optimized out>, context=<optimized out>) at jsruntime/qv4function.cpp:68
#61 0x00007f35acf3ad77 in QQmlJavaScriptExpression::evaluate (this=this@entry=0x55cdb906c390, callData=callData@entry=0x7f358bbc14d0, isUndefined=isUndefined@entry=0x0) at ../../include/QtQml/5.12.2/QtQml/private/../../../../../src/qml/jsruntime/qv4value_p.h:177
#62 0x00007f35acedbcbf in QQmlBoundSignalExpression::evaluate (this=<optimized out>, a=0x0) at ../../include/QtQml/5.12.2/QtQml/private/../../../../../src/qml/jsruntime/qv4jscall_p.h:95
#63 0x00007f35acedce2b in QQmlBoundSignal_callback (e=0x55cdb906c340, a=0x0) at ../../include/QtQml/5.12.2/QtQml/private/../../../../../src/qml/qml/qqmlboundsignalexpressionpointer_p.h:69
#64 0x00007f35acf1c803 in QQmlNotifier::emitNotify (endpoint=<optimized out>, a=a@entry=0x0) at qml/qqmlnotifier.cpp:104
#65 0x00007f35acebeada in QQmlData::signalEmitted (object=0x55cdb90669a0, index=64, a=0x0) at qml/qqmlengine.cpp:883
#66 0x00007f35ade29d5f in QMetaObject::activate (sender=0x55cdb90669a0, signalOffset=<optimized out>, local_signal_index=<optimized out>, argv=0x0) at ../../include/QtCore/../../src/corelib/kernel/qobject.h:121
#67 0x00007f35a7480ab8 in QQuickAbstractButtonPrivate::handleRelease (this=0x55cdb9066a60, point=...) at qquickabstractbutton.cpp:179
#68 0x00007f35a749db05 in QQuickControl::mouseReleaseEvent (this=<optimized out>, event=0x7ffebe758130) at /usr/include/x86_64-linux-gnu/qt5/QtGui/qevent.h:128
#69 0x00007f35ad302938 in QQuickItem::event (this=0x55cdb90669a0, ev=0x7ffebe758130) at items/qquickitem.cpp:8038
#70 0x00007f35ae860551 in QApplicationPrivate::notify_helper (this=this@entry=0x55cdb788eac0, receiver=receiver@entry=0x55cdb90669a0, e=e@entry=0x7ffebe758130) at kernel/qapplication.cpp:3736
#71 0x00007f35ae867930 in QApplication::notify (this=0x7ffebe759070, receiver=0x55cdb90669a0, e=0x7ffebe758130) at kernel/qapplication.cpp:3483
#72 0x00007f35ade008e9 in QCoreApplication::notifyInternal2 (receiver=0x55cdb90669a0, event=0x7ffebe758130) at ../../include/QtCore/5.12.2/QtCore/private/../../../../../src/corelib/thread/qthread_p.h:322
#73 0x00007f35ad31c585 in QQuickWindowPrivate::deliverMouseEvent (this=this@entry=0x55cdb7a5f5a0, pointerEvent=0x55cdb926f360) at items/qquickwindow.cpp:1784
#74 0x00007f35ad31d7db in QQuickWindowPrivate::deliverPointerEvent (this=this@entry=0x55cdb7a5f5a0, event=0x55cdb926f360) at items/qquickwindow.cpp:2347
#75 0x00007f35ad31e51f in QQuickWindowPrivate::handleMouseEvent (event=0x7ffebe758130, this=0x55cdb7a5f5a0) at items/qquickwindow.cpp:2210
#76 QQuickWindowPrivate::handleMouseEvent (this=0x55cdb7a5f5a0, event=0x7ffebe758130) at items/qquickwindow.cpp:2163
#77 0x00007f35ae27a1e5 in QWindow::event (this=this@entry=0x55cdb7c248d0, ev=ev@entry=0x7ffebe758130) at kernel/qwindow.cpp:2258
#78 0x00007f35ad31f8ab in QQuickWindow::event (this=0x55cdb7c248d0, e=0x7ffebe758130) at items/qquickwindow.cpp:1673
#79 0x00007f35ae860551 in QApplicationPrivate::notify_helper (this=this@entry=0x55cdb788eac0, receiver=receiver@entry=0x55cdb7c248d0, e=e@entry=0x7ffebe758130) at kernel/qapplication.cpp:3736
#80 0x00007f35ae867930 in QApplication::notify (this=0x7ffebe759070, receiver=0x55cdb7c248d0, e=0x7ffebe758130) at kernel/qapplication.cpp:3483
#81 0x00007f35ade008e9 in QCoreApplication::notifyInternal2 (receiver=0x55cdb7c248d0, event=0x7ffebe758130) at ../../include/QtCore/5.12.2/QtCore/private/../../../../../src/corelib/thread/qthread_p.h:322
#82 0x00007f35ad5cfcb7 in QQuickWidget::mouseReleaseEvent (this=<optimized out>, e=0x7ffebe758760) at qquickwidget.cpp:1372
#83 0x00007f35ae89fb58 in QWidget::event (this=this@entry=0x55cdb7c3e540, event=event@entry=0x7ffebe758760) at kernel/qwidget.cpp:8955
#84 0x00007f35ad5cfe55 in QQuickWidget::event (this=0x55cdb7c3e540, e=0x7ffebe758760) at qquickwidget.cpp:1525
#85 0x00007f35ae860551 in QApplicationPrivate::notify_helper (this=this@entry=0x55cdb788eac0, receiver=receiver@entry=0x55cdb7c3e540, e=e@entry=0x7ffebe758760) at kernel/qapplication.cpp:3736
#86 0x00007f35ae867b77 in QApplication::notify (this=<optimized out>, receiver=0x55cdb7c3e540, e=0x7ffebe758760) at kernel/qapplication.cpp:3197
#87 0x00007f35ade008e9 in QCoreApplication::notifyInternal2 (receiver=0x55cdb7c3e540, event=0x7ffebe758760) at ../../include/QtCore/5.12.2/QtCore/private/../../../../../src/corelib/thread/qthread_p.h:322
#88 0x00007f35ae866ebf in QApplicationPrivate::sendMouseEvent (receiver=receiver@entry=0x55cdb7c3e540, event=event@entry=0x7ffebe758760, alienWidget=0x55cdb7c3e540, nativeWidget=0x55cdb794ebd0, buttonDown=buttonDown@entry=0x7f35aed618d0 <qt_button_down>, lastMouseReceiver=..., spontaneous=true, onlyDispatchEnterLeave=false) at kernel/qapplication.cpp:2683
#89 0x00007f35ae8ba57b in QWidgetWindow::handleMouseEvent (this=0x55cdb7a280c0, event=0x7ffebe758be0) at /usr/include/c++/8/bits/atomic_base.h:390
#90 0x00007f35ae8bd53f in QWidgetWindow::event (event=0x7ffebe758be0, this=0x55cdb7a280c0) at kernel/qwidgetwindow.cpp:281
#91 QWidgetWindow::event (this=0x55cdb7a280c0, event=0x7ffebe758be0) at kernel/qwidgetwindow.cpp:224
#92 0x00007f35ae860551 in QApplicationPrivate::notify_helper (this=this@entry=0x55cdb788eac0, receiver=receiver@entry=0x55cdb7a280c0, e=e@entry=0x7ffebe758be0) at kernel/qapplication.cpp:3736
#93 0x00007f35ae867930 in QApplication::notify (this=0x7ffebe759070, receiver=0x55cdb7a280c0, e=0x7ffebe758be0) at kernel/qapplication.cpp:3483
#94 0x00007f35ade008e9 in QCoreApplication::notifyInternal2 (receiver=0x55cdb7a280c0, event=0x7ffebe758be0) at ../../include/QtCore/5.12.2/QtCore/private/../../../../../src/corelib/thread/qthread_p.h:322
#95 0x00007f35ae26ec6c in QGuiApplicationPrivate::processMouseEvent (e=0x55cdb97a3fa0) at kernel/qguiapplication.cpp:2102
#96 0x00007f35ae270075 in QGuiApplicationPrivate::processWindowSystemEvent (e=e@entry=0x55cdb97a3fa0) at kernel/qguiapplication.cpp:1837
#97 0x00007f35ae24a05b in QWindowSystemInterface::sendWindowSystemEvents (flags=flags@entry=...) at kernel/qwindowsysteminterface.cpp:1071
#98 0x00007f35a8c0767a in xcbSourceDispatch (source=<optimized out>) at qxcbeventdispatcher.cpp:105
#99 0x00007f35ab624aae in g_main_dispatch (context=0x7f35a0004fd0) at ../../../glib/gmain.c:3189
#100 g_main_context_dispatch (context=context@entry=0x7f35a0004fd0) at ../../../glib/gmain.c:3854
#101 0x00007f35ab624d48 in g_main_context_iterate (context=context@entry=0x7f35a0004fd0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../../../glib/gmain.c:3927
#102 0x00007f35ab624ddc in g_main_context_iteration (context=0x7f35a0004fd0, may_block=may_block@entry=1) at ../../../glib/gmain.c:3988
#103 0x00007f35ade54047 in QEventDispatcherGlib::processEvents (this=0x55cdb78e7520, flags=...) at kernel/qeventdispatcher_glib.cpp:422
#104 0x00007f35addff5bb in QEventLoop::exec (this=this@entry=0x7ffebe758f90, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:140
#105 0x00007f35ade075e2 in QCoreApplication::exec () at ../../include/QtCore/../../src/corelib/global/qflags.h:120
#106 0x000055cdb6043639 in main (argc=<optimized out>, argv=<optimized out>) at ./app/main.cpp:71
[Inferior 1 (process 5549) detached]

I can also replicate this in OpenSuse Tumbleweed with Plasma 5.16 beta packages.

*** This bug has been marked as a duplicate of bug 408325 ***