Do not attempt to compare iterators of potentially different containers
I got this crash after leaving my system unattended:
0 std::__atomic_base<int>::load(std::memory_order) const (this=0x0, __m=std::memory_order_relaxed) at /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/bits/atomic_base.h:426
1 QAtomicOps<int>::loadRelaxed<int>(std::atomic<int> const&) (_q_value=...) at /home/apol/devel/kde5/include/QtCore/qatomic_cxx11.h:239
2 QBasicAtomicInteger<int>::loadRelaxed() const (this=0x0) at /home/apol/devel/kde5/include/QtCore/qbasicatomic.h:107
3 QtPrivate::RefCount::isShared() const (this=0x0) at /home/apol/devel/kde5/include/QtCore/qrefcount.h:101
4 QMap<KWin::Group*, KWin::Layer>::detach() (this=0x7f1b577b7870 <qt_array+48>) at /home/apol/devel/kde5/include/QtCore/qmap.h:357
5 QMap<KWin::Group*, KWin::Layer>::find(KWin::Group* const&) (this=0x7f1b577b7870 <qt_array+48>, akey=<optimized out>) at /home/apol/devel/kde5/include/QtCore/qmap.h:861
6 KWin::Workspace::constrainedStackingOrder() (this=this@entry=0x564b368c6290) at /home/apol/devel/frameworks/kwin/src/layers.cpp:502
7 0x00007f1b5708d027 in KWin::Workspace::updateStackingOrder(bool) (this=0x564b368c6290, propagate_new_clients=false) at /home/apol/devel/frameworks/kwin/src/layers.cpp:110
8 0x00007f1b571629be in KWin::X11Client::setFrameGeometry(QRect const&, KWin::AbstractClient::ForceGeometry_t) (this=0x564b374b4e60, rect=<optimized out>, force=<optimized out>) at /home/apol/devel/frameworks/kwin/src/x11client.cpp:4193
9 0x00007f1b57003fbf in KWin::AbstractClient::blockGeometryUpdates(bool) (this=this@entry=0x564b374b4e60, block=<optimized out>) at /home/apol/devel/frameworks/kwin/src/abstract_client.cpp:871
10 0x00007f1b570054f1 in KWin::GeometryUpdatesBlocker::~GeometryUpdatesBlocker() (this=<optimized out>) at /home/apol/devel/frameworks/kwin/src/abstract_client.h:1334
11 KWin::AbstractClient::checkWorkspacePosition(QRect, int, QRect) (this=0x564b374b4e60, oldGeometry=..., oldDesktop=1, oldClientGeometry=...) at /home/apol/devel/frameworks/kwin/src/abstract_client.cpp:3189
12 0x00007f1b571409e0 in KWin::Workspace::updateClientArea(bool) (this=<optimized out>, this@entry=0x564b368c6290, force=false) at /home/apol/devel/frameworks/kwin/src/workspace.cpp:2234
13 0x00007f1b57138fc6 in KWin::Workspace::updateClientArea() (this=0x564b368c6290) at /home/apol/devel/frameworks/kwin/src/workspace.cpp:2243
14 KWin::Workspace::desktopResized() (this=0x564b368c6290) at /home/apol/devel/frameworks/kwin/src/workspace.cpp:1980
15 0x00007f1b57707806 in QtPrivate::QSlotObjectBase::call(QObject*, void**) (a=0x7fff671a2e50, r=0x564b368c6290, this=0x564b372f1860) at ../../include/QtCore/../../../../../devel/frameworks/qt5/qtbase/src/corelib/kernel/qobjectdefs_impl.h:398
16 doActivate<false>(QObject*, int, void**) (sender=0x564b368a0fe0, signal_index=4, argv=0x7fff671a2e50) at /home/apol/devel/frameworks/qt5/qtbase/src/corelib/kernel/qobject.cpp:3886
17 0x00007f1b57707806 in QtPrivate::QSlotObjectBase::call(QObject*, void**) (a=0x7fff671a2f30, r=0x564b368a0fe0, this=0x564b368a10f0) at ../../include/QtCore/../../../../../devel/frameworks/qt5/qtbase/src/corelib/kernel/qobjectdefs_impl.h:398
18 doActivate<false>(QObject*, int, void**) (sender=0x564b36775640, signal_index=3, argv=0x7fff671a2f30) at /home/apol/devel/frameworks/qt5/qtbase/src/corelib/kernel/qobject.cpp:3886
19 0x00007f1b4ee2f9c3 in KWin::DrmBackend::updateOutputs() (this=this@entry=0x564b36775640) at /home/apol/devel/frameworks/kwin/src/plugins/platforms/drm/drm_backend.cpp:374
20 0x00007f1b4ee31d54 in KWin::DrmBackend::openDrm()::$_2::operator()() const (this=<optimized out>) at /home/apol/devel/frameworks/kwin/src/plugins/platforms/drm/drm_backend.cpp:328