SUMMARY
When we go to the home page (home button in the sidebar) without applying changed settings and then choose to apply the setting upon closing, 'System Settings crashes.

STEPS TO REPRODUCE
1. Change any setting (do not press 'apply')
2. Press the 'back' button in the sidebar
3. Press the 'home' button in the sidebar
4. Close System Settings
5. Press 'apply' when the dialog pops up asking you if you want to apply changed settings.

OBSERVED RESULT
We get a notification that system settings has crashed.

EXPECTED RESULT
System settings should exit normally after applying the changed settings

SOFTWARE/OS VERSIONS
System Settings version: 5.21.5 (I could not find it in the list of versions)
Operating System: KDE neon 5.21
KDE Plasma Version: 5.21.5
KDE Frameworks Version: 5.81.0
Qt Version: 5.15.2
Kernel Version: 5.4.0-72-generic
OS Type: 64-bit
Graphics Platform: X11

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

[New LWP 1080]
[New LWP 1081]
[New LWP 1082]
[New LWP 1083]
[New LWP 1084]
[New LWP 1085]
[New LWP 1100]
[New LWP 1110]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
0x00007fdf04fd4aff in __GI___poll (fds=0x7ffe2a932f68, nfds=1, timeout=1000) at ../sysdeps/unix/sysv/linux/poll.c:29
[Current thread is 1 (Thread 0x7fdf00d90500 (LWP 1078))]

Thread 9 (Thread 0x7fdeda256700 (LWP 1110)):
#0  0x00007fdf04fd4aff in __GI___poll (fds=0x7fdec4007170, nfds=3, timeout=22223) at ../sysdeps/unix/sysv/linux/poll.c:29
#1  0x00007fdee8ca91d6 in ?? () from /lib/x86_64-linux-gnu/libpulse.so.0
#2  0x00007fdee8c9a841 in pa_mainloop_poll () from /lib/x86_64-linux-gnu/libpulse.so.0
#3  0x00007fdee8c9aec3 in pa_mainloop_iterate () from /lib/x86_64-linux-gnu/libpulse.so.0
#4  0x00007fdee8c9af70 in pa_mainloop_run () from /lib/x86_64-linux-gnu/libpulse.so.0
#5  0x00007fdee8ca911d in ?? () from /lib/x86_64-linux-gnu/libpulse.so.0
#6  0x00007fdee8c4572c in ?? () from /usr/lib/x86_64-linux-gnu/pulseaudio/libpulsecommon-13.99.so
#7  0x00007fdf03b61609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#8  0x00007fdf04fe1293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 8 (Thread 0x7fdedbfff700 (LWP 1100)):
#0  0x00007fdf04ee6444 in __GI___errno_location () at errno-loc.c:26
#1  0x00007fdf031a2375 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#2  0x00007fdf031a24a3 in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007fdf056eeffb in QEventDispatcherGlib::processEvents (this=0x7fded0000b60, flags=...) at kernel/qeventdispatcher_glib.cpp:425
#4  0x00007fdf056931eb in QEventLoop::exec (this=this@entry=0x7fdedbffecc0, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:141
#5  0x00007fdf054ada52 in QThread::exec (this=<optimized out>) at ../../include/QtCore/../../src/corelib/global/qflags.h:121
#6  0x00007fdf047a2fa9 in ?? () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#7  0x00007fdf054aebec in QThreadPrivate::start (arg=0x5583794e3eb0) at thread/qthread_unix.cpp:329
#8  0x00007fdf03b61609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#9  0x00007fdf04fe1293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 7 (Thread 0x7fdef49d3700 (LWP 1085)):
#0  0x00007fdf031a02f0 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#1  0x00007fdf031a182b in g_main_context_prepare () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#2  0x00007fdf031a229b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007fdf031a24a3 in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#4  0x00007fdf056eeffb in QEventDispatcherGlib::processEvents (this=0x7fded4000b60, flags=...) at kernel/qeventdispatcher_glib.cpp:425
#5  0x00007fdf056931eb in QEventLoop::exec (this=this@entry=0x7fdef49d2cc0, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:141
#6  0x00007fdf054ada52 in QThread::exec (this=<optimized out>) at ../../include/QtCore/../../src/corelib/global/qflags.h:121
#7  0x00007fdf047a2fa9 in ?? () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#8  0x00007fdf054aebec in QThreadPrivate::start (arg=0x5583787c08e0) at thread/qthread_unix.cpp:329
#9  0x00007fdf03b61609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#10 0x00007fdf04fe1293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 6 (Thread 0x7fdef61dd700 (LWP 1084)):
#0  0x00007ffe2a948933 in clock_gettime ()
#1  0x00007fdf04f9f235 in __GI___clock_gettime (clock_id=clock_id@entry=1, tp=tp@entry=0x7fdef61dca30) at ../sysdeps/unix/sysv/linux/clock_gettime.c:38
#2  0x00007fdf056ee8c5 in qt_clock_gettime (ts=0x7fdef61dca30, clock=1) at kernel/qelapsedtimer_unix.cpp:175
#3  do_gettime (frac=<synthetic pointer>, sec=<synthetic pointer>) at kernel/qelapsedtimer_unix.cpp:166
#4  qt_gettime () at kernel/qelapsedtimer_unix.cpp:175
#5  0x00007fdf056ed15d in QTimerInfoList::updateCurrentTime (this=this@entry=0x7fdee0004d50) at kernel/qtimerinfo_unix.cpp:91
#6  0x00007fdf056ed739 in QTimerInfoList::timerWait (this=0x7fdee0004d50, tm=...) at kernel/qtimerinfo_unix.cpp:388
#7  0x00007fdf056eed56 in timerSourcePrepareHelper (timeout=0x7fdef61dcaf4, src=<optimized out>) at kernel/qeventdispatcher_glib.cpp:162
#8  timerSourcePrepare (source=<optimized out>, timeout=0x7fdef61dcaf4) at kernel/qeventdispatcher_glib.cpp:166
#9  0x00007fdf031a18ef in g_main_context_prepare () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#10 0x00007fdf031a229b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#11 0x00007fdf031a24a3 in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#12 0x00007fdf056eeffb in QEventDispatcherGlib::processEvents (this=0x7fdee0000b60, flags=...) at kernel/qeventdispatcher_glib.cpp:425
#13 0x00007fdf056931eb in QEventLoop::exec (this=this@entry=0x7fdef61dccc0, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:141
#14 0x00007fdf054ada52 in QThread::exec (this=<optimized out>) at ../../include/QtCore/../../src/corelib/global/qflags.h:121
#15 0x00007fdf047a2fa9 in ?? () from /lib/x86_64-linux-gnu/libQt5Qml.so.5
#16 0x00007fdf054aebec in QThreadPrivate::start (arg=0x5583784fa140) at thread/qthread_unix.cpp:329
#17 0x00007fdf03b61609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#18 0x00007fdf04fe1293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 5 (Thread 0x7fdef6c21700 (LWP 1083)):
#0  futex_abstimed_wait_cancelable (private=<optimized out>, abstime=0x7fdef6c20c50, clockid=<optimized out>, expected=0, futex_word=0x5583782ff080) at ../sysdeps/nptl/futex-internal.h:320
#1  __pthread_cond_wait_common (abstime=0x7fdef6c20c50, clockid=<optimized out>, mutex=0x5583782ff030, cond=0x5583782ff058) at pthread_cond_wait.c:520
#2  __pthread_cond_timedwait (cond=0x5583782ff058, mutex=0x5583782ff030, abstime=0x7fdef6c20c50) at pthread_cond_wait.c:656
#3  0x00007fdf054b4ce8 in QWaitConditionPrivate::wait_relative (this=0x5583782ff030, deadline=...) at thread/qwaitcondition_unix.cpp:136
#4  QWaitConditionPrivate::wait (deadline=..., this=0x5583782ff030) at thread/qwaitcondition_unix.cpp:144
#5  QWaitCondition::wait (this=this@entry=0x5583781df510, mutex=mutex@entry=0x5583783453a8, deadline=...) at thread/qwaitcondition_unix.cpp:225
#6  0x00007fdf054b21f1 in QThreadPoolThread::run (this=0x5583781df500) at ../../include/QtCore/../../src/corelib/thread/qmutex.h:270
#7  0x00007fdf054aebec in QThreadPrivate::start (arg=0x5583781df500) at thread/qthread_unix.cpp:329
#8  0x00007fdf03b61609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#9  0x00007fdf04fe1293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 4 (Thread 0x7fdef7422700 (LWP 1082)):
#0  futex_abstimed_wait_cancelable (private=<optimized out>, abstime=0x7fdef7421c50, clockid=<optimized out>, expected=0, futex_word=0x55837830f150) at ../sysdeps/nptl/futex-internal.h:320
#1  __pthread_cond_wait_common (abstime=0x7fdef7421c50, clockid=<optimized out>, mutex=0x55837830f100, cond=0x55837830f128) at pthread_cond_wait.c:520
#2  __pthread_cond_timedwait (cond=0x55837830f128, mutex=0x55837830f100, abstime=0x7fdef7421c50) at pthread_cond_wait.c:656
#3  0x00007fdf054b4ce8 in QWaitConditionPrivate::wait_relative (this=0x55837830f100, deadline=...) at thread/qwaitcondition_unix.cpp:136
#4  QWaitConditionPrivate::wait (deadline=..., this=0x55837830f100) at thread/qwaitcondition_unix.cpp:144
#5  QWaitCondition::wait (this=this@entry=0x5583782f53c0, mutex=mutex@entry=0x5583783453a8, deadline=...) at thread/qwaitcondition_unix.cpp:225
#6  0x00007fdf054b21f1 in QThreadPoolThread::run (this=0x5583782f53b0) at ../../include/QtCore/../../src/corelib/thread/qmutex.h:270
#7  0x00007fdf054aebec in QThreadPrivate::start (arg=0x5583782f53b0) at thread/qthread_unix.cpp:329
#8  0x00007fdf03b61609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#9  0x00007fdf04fe1293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 3 (Thread 0x7fdeff023700 (LWP 1081)):
#0  0x00007fdf031ef4dd in g_mutex_lock () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#1  0x00007fdf031a178b in g_main_context_prepare () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#2  0x00007fdf031a229b in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007fdf031a24a3 in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#4  0x00007fdf056eeffb in QEventDispatcherGlib::processEvents (this=0x7fdef0000b60, flags=...) at kernel/qeventdispatcher_glib.cpp:425
#5  0x00007fdf056931eb in QEventLoop::exec (this=this@entry=0x7fdeff022cb0, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:141
#6  0x00007fdf054ada52 in QThread::exec (this=<optimized out>) at ../../include/QtCore/../../src/corelib/global/qflags.h:121
#7  0x00007fdf043bef4b in ?? () from /lib/x86_64-linux-gnu/libQt5DBus.so.5
#8  0x00007fdf054aebec in QThreadPrivate::start (arg=0x7fdf04442d80) at thread/qthread_unix.cpp:329
#9  0x00007fdf03b61609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#10 0x00007fdf04fe1293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 2 (Thread 0x7fdf001cb700 (LWP 1080)):
#0  0x00007fdf04fd4aff in __GI___poll (fds=0x7fdf001cabe8, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
#1  0x00007fdf03ba8c1a in ?? () from /lib/x86_64-linux-gnu/libxcb.so.1
#2  0x00007fdf03baa90a in xcb_wait_for_event () from /lib/x86_64-linux-gnu/libxcb.so.1
#3  0x00007fdf00708d58 in QXcbEventQueue::run (this=0x558378023d80) at qxcbeventqueue.cpp:228
#4  0x00007fdf054aebec in QThreadPrivate::start (arg=0x558378023d80) at thread/qthread_unix.cpp:329
#5  0x00007fdf03b61609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#6  0x00007fdf04fe1293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Thread 1 (Thread 0x7fdf00d90500 (LWP 1078)):
[KCrash Handler]
#4  MenuItem::children (this=0x0) at ./core/MenuItem.cpp:99
#5  0x00007fdef62fc8be in SidebarMode::updateDefaults (this=0x5583784a20d0) at ./sidebar/SidebarMode.cpp:622
#6  0x00007fdf056cb6fe in QtPrivate::QSlotObjectBase::call (a=0x7ffe2a9338e0, r=0x5583784a20d0, this=0x5583784d5c90) at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:398
#7  doActivate<false> (sender=0x5583784786e0, signal_index=8, argv=0x7ffe2a9338e0) at kernel/qobject.cpp:3886
#8  0x00007fdf056c4ac7 in QMetaObject::activate (sender=sender@entry=0x5583784786e0, m=m@entry=0x7fdef62078c0 <ModuleView::staticMetaObject>, local_signal_index=local_signal_index@entry=1, argv=argv@entry=0x0) at kernel/qobject.cpp:3946
#9  0x00007fdef61f2e07 in ModuleView::moduleSaved (this=this@entry=0x5583784786e0) at ./obj-x86_64-linux-gnu/core/systemsettingsview_autogen/EWIEGA46WW/moc_ModuleView.cpp:224
#10 0x0000558376875ae7 in ModuleView::moduleSave (module=0x5583794bf870, this=0x5583784786e0) at ./core/ModuleView.cpp:394
#11 ModuleView::resolveChanges (currentProxy=0x5583794bf870, this=0x5583784786e0) at ./core/ModuleView.cpp:354
#12 ModuleView::resolveChanges (this=0x5583784786e0, currentProxy=0x5583794bf870) at ./core/ModuleView.cpp:337
#13 0x0000558376862e68 in SettingsBase::queryClose (this=0x5583780f66b0) at ./app/SettingsBase.cpp:352
#14 0x00007fdf06f0882d in KMainWindow::closeEvent (this=0x5583780f66b0, e=0x7ffe2a933e40) at ./src/kmainwindow.cpp:544
#15 0x00007fdf062d8c06 in QWidget::event (this=this@entry=0x5583780f66b0, event=event@entry=0x7ffe2a933e40) at kernel/qwidget.cpp:8663
#16 0x00007fdf063f18c8 in QMainWindow::event (this=this@entry=0x5583780f66b0, event=event@entry=0x7ffe2a933e40) at widgets/qmainwindow.cpp:1341
#17 0x00007fdf06f08c03 in KMainWindow::event (this=this@entry=0x5583780f66b0, ev=ev@entry=0x7ffe2a933e40) at ./src/kmainwindow.cpp:908
#18 0x00007fdf06f5418b in KXmlGuiWindow::event (this=0x5583780f66b0, ev=0x7ffe2a933e40) at ./src/kxmlguiwindow.cpp:110
#19 0x00007fdf06295dc3 in QApplicationPrivate::notify_helper (this=<optimized out>, receiver=0x5583780f66b0, e=0x7ffe2a933e40) at kernel/qapplication.cpp:3632
#20 0x00007fdf0629ebb8 in QApplication::notify(QObject*, QEvent*) () at kernel/qapplication.cpp:3156
#21 0x00007fdf056946da in QCoreApplication::notifyInternal2 (receiver=0x5583780f66b0, event=0x7ffe2a933e40) at ../../include/QtCore/5.15.2/QtCore/private/../../../../../src/corelib/thread/qthread_p.h:325
#22 0x00007fdf062d41d8 in QWidgetPrivate::close_helper (mode=QWidgetPrivate::CloseWithSpontaneousEvent, this=0x558378030d50) at kernel/qwidget.cpp:8238
#23 QWidgetPrivate::close_helper (this=0x558378030d50, mode=mode@entry=QWidgetPrivate::CloseWithSpontaneousEvent) at kernel/qwidget.cpp:8223
#24 0x00007fdf062f36d2 in QWidgetWindow::handleCloseEvent (this=<optimized out>, event=0x7ffe2a934180) at ../../include/QtWidgets/../../src/widgets/kernel/qwidget.h:132
#25 0x00007fdf062f7408 in QWidgetWindow::event (event=0x7ffe2a934180, this=0x55837834ff90) at kernel/qwidgetwindow.cpp:256
#26 QWidgetWindow::event (this=0x55837834ff90, event=0x7ffe2a934180) at kernel/qwidgetwindow.cpp:239
#27 0x00007fdf06295dc3 in QApplicationPrivate::notify_helper (this=<optimized out>, receiver=0x55837834ff90, e=0x7ffe2a934180) at kernel/qapplication.cpp:3632
#28 0x00007fdf0629ebb8 in QApplication::notify(QObject*, QEvent*) () at kernel/qapplication.cpp:3156
#29 0x00007fdf056946da in QCoreApplication::notifyInternal2 (receiver=0x55837834ff90, event=0x7ffe2a934180) at ../../include/QtCore/5.15.2/QtCore/private/../../../../../src/corelib/thread/qthread_p.h:325
#30 0x00007fdf05b9d395 in QGuiApplicationPrivate::processCloseEvent(QWindowSystemInterfacePrivate::CloseEvent*) () at /usr/include/c++/9/bits/atomic_base.h:413
#31 0x00007fdf05ba1b7c in QGuiApplicationPrivate::processWindowSystemEvent (e=0x55837975f510) at kernel/qguiapplication.cpp:2046
#32 0x00007fdf05b797ac in QWindowSystemInterface::sendWindowSystemEvents (flags=flags@entry=...) at kernel/qwindowsysteminterface.cpp:1169
#33 0x00007fdf00709d7e in xcbSourceDispatch (source=<optimized out>) at qxcbeventdispatcher.cpp:105
#34 0x00007fdf031a217d in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#35 0x00007fdf031a2400 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#36 0x00007fdf031a24a3 in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#37 0x00007fdf056eefe2 in QEventDispatcherGlib::processEvents (this=0x5583780e7910, flags=...) at kernel/qeventdispatcher_glib.cpp:423
#38 0x00007fdf056931eb in QEventLoop::exec (this=this@entry=0x7ffe2a934440, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:141
#39 0x00007fdf0569b394 in QCoreApplication::exec () at ../../include/QtCore/../../src/corelib/global/qflags.h:121
#40 0x0000558376861392 in main (argc=<optimized out>, argv=<optimized out>) at ./app/main.cpp:222
[Inferior 1 (process 1078) detached]