This is the backtrace I've managed to get (sorry for some missing debug symbols, thanks Arch -.-)...

SIGSEGV plasmashell

#0  0x0000559e4184ce80 in  ()
#1  0x00007f5420488b29 in QMetaObject::cast(QObject const*) const () at /usr/lib/libQt5Core.so.5
#2  0x00007f5423a45f70 in qobject_cast<QAction*>(QObject*) (object=0x559e42926920)
    at /usr/include/qt/QtCore/qobject.h:514
#3  0x00007f5423a45f70 in KActionConflictDetector::eventFilter(QObject*, QEvent*) (this=0x559e400dadf0, watched=0x559e42926920, event=0x559e42aafce0)
    at /tmp/kxmlgui/repos/extra-x86_64/src/kxmlgui-5.41.0/src/kactionconflictdetector.cpp:44
#4  0x00007f542047ed5c in QCoreApplicationPrivate::sendThroughApplicationEventFilters(QObject*, QEvent*) ()
    at /usr/lib/libQt5Core.so.5
#5  0x00007f5421483e7a in QApplicationPrivate::notify_helper(QObject*, QEvent*) ()
    at /usr/lib/libQt5Widgets.so.5
#6  0x00007f542148b816 in QApplication::notify(QObject*, QEvent*) () at /usr/lib/libQt5Widgets.so.5
#7  0x00007f542047f1e0 in QCoreApplication::notifyInternal2(QObject*, QEvent*) ()
    at /usr/lib/libQt5Core.so.5
#8  0x00007f5420481e46 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) ()
    at /usr/lib/libQt5Core.so.5
#9  0x00007f54204dc0a4 in  () at /usr/lib/libQt5Core.so.5
#10 0x00007f541a43be98 in g_main_context_dispatch () at /usr/lib/libglib-2.0.so.0
#11 0x00007f541a43c0e1 in  () at /usr/lib/libglib-2.0.so.0
#12 0x00007f541a43c16e in g_main_context_iteration () at /usr/lib/libglib-2.0.so.0
#13 0x00007f54204db691 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/libQt5Core.so.5
#14 0x00007f54126b1282 in  () at /usr/lib/libQt5XcbQpa.so.5
#15 0x00007f542047d82b in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/libQt5Core.so.5
#16 0x00007f5420486b18 in QCoreApplication::exec() () at /usr/lib/libQt5Core.so.5
#17 0x0000559e3ec288d8 in main(int, char**) (argc=<optimized out>, argv=<optimized out>)
    at /tmp/plasma-workspace/src/plasma-workspace-5.11.5/shell/main.cpp:167