I can not reproduce it, the dpy=0x18 doesn't make much sense :/

I don't think i was doing something special, maybe installing some packages with the arch command line tool?

#6  0x00007fe0e2af41ed in XPending (dpy=0x18) at /usr/src/debug/libx11/libX11-1.8.6/src/Pending.c:51
#7  0x00007fe0c846bd91 in XlibNotifications::processEvents() (this=<optimized out>) at /usr/src/debug/plasma-desktop/plasma-desktop-5.27.7.1/kcms/touchpad/backends/x11/xlibnotifications.cpp:63
#8  XlibNotifications::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (_o=0x55f7b3392af0, _c=<optimized out>, _id=<optimized out>, _a=<optimized out>) at /usr/src/debug/plasma-desktop/build/kcms/touchpad/touchpad_autogen/include/moc_xlibnotifications.cpp:91
#9  0x00007fe0e32d17f3 in doActivate<false>(QObject*, int, void**) (sender=0x55f7b33acd40, signal_index=5, argv=0x7fffe3e754e0) at kernel/qobject.cpp:3937
#10 0x00007fe0e32d2ea9 in QSocketNotifier::activated(int, QSocketNotifier::QPrivateSignal) (this=this@entry=0x55f7b33acd40, _t1=<optimized out>, _t2=...) at .moc/moc_qsocketnotifier.cpp:185
#11 0x00007fe0e32d2f9b in QSocketNotifier::event(QEvent*) (this=0x55f7b33acd40, e=<optimized out>) at kernel/qsocketnotifier.h:115
#12 0x00007fe0e3f7893f in QApplicationPrivate::notify_helper(QObject*, QEvent*) (this=<optimized out>, receiver=0x55f7b33acd40, e=0x7fffe3e755f0) at kernel/qapplication.cpp:3640
#13 0x00007fe0e329c6f8 in QCoreApplication::notifyInternal2(QObject*, QEvent*) (receiver=0x55f7b33acd40, event=0x7fffe3e755f0) at kernel/qcoreapplication.cpp:1064
#14 0x00007fe0e32ea276 in socketNotifierSourceDispatch(GSource*, GSourceFunc, gpointer) (source=0x55f7b30eb9c0) at kernel/qeventdispatcher_glib.cpp:107
#15 0x00007fe0e1d0fa31 in g_main_dispatch (context=0x7fe0d8000ee0) at ../glib/glib/gmain.c:3460
#16 g_main_context_dispatch (context=0x7fe0d8000ee0) at ../glib/glib/gmain.c:4200
#17 0x00007fe0e1d6ccc9 in g_main_context_iterate.isra.0 (context=context@entry=0x7fe0d8000ee0, block=block@entry=1, dispatch=dispatch@entry=1, self=<optimized out>) at ../glib/glib/gmain.c:4276
#18 0x00007fe0e1d0d0e2 in g_main_context_iteration (context=0x7fe0d8000ee0, may_block=1) at ../glib/glib/gmain.c:4343
#19 0x00007fe0e32eb51c in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (this=0x55f7b30f0020, flags=...) at kernel/qeventdispatcher_glib.cpp:423
#20 0x00007fe0e329b404 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (this=this@entry=0x7fffe3e75870, flags=..., flags@entry=...) at ../../include/QtCore/../../src/corelib/global/qflags.h:69
#21 0x00007fe0e329c8a3 in QCoreApplication::exec() () at ../../include/QtCore/../../src/corelib/global/qflags.h:121
#22 0x00007fe0e373bef2 in QGuiApplication::exec() () at kernel/qguiapplication.cpp:1870
#23 0x00007fe0e3f76cda in QApplication::exec() () at kernel/qapplication.cpp:2832
#24 0x000055f7b1300a2b in main(int, char**) (argc=<optimized out>, argv=<optimized out>) at /usr/src/debug/kded/kded-5.108.0/src/kded.cpp:785
[Inferior 1 (process 975) detached]