SUMMARY

STEPS TO REPRODUCE
1. No idea how to reproduce

OBSERVED RESULT
Crash

EXPECTED RESULT
No crash ;)

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 5.27 build from git 9f46f605184976fe025c75b3a5452b4b0ed4c23c (yeah i know it's quite old but might still be helfpful)
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: master (but more recent)
Qt Version: 5.15.9

ADDITIONAL INFORMATION

Backtrace

#0  0x000000000048dc46 in KWin::Window::clientGeometry() const (this=0x0) at /home/carl/kde/src/kwin/src/window.h:2080
#1  0x000000000049dbe2 in KWin::WindowScreenCastSource::textureSize() const (this=0x30b9060) at /home/carl/kde/src/kwin/src/plugins/screencast/windowscreencastsource.cpp:40
#2  0x0000000000495bcd in KWin::ScreenCastStream::recordFrame(QRegion const&) (this=0x3325120, _damagedRegion=...)
    at /home/carl/kde/src/kwin/src/plugins/screencast/screencaststream.cpp:429
#3  0x0000000000494b0e in operator()() const (__closure=0x2ebc410) at /home/carl/kde/src/kwin/src/plugins/screencast/screencaststream.cpp:308
#4  0x000000000049855c in QtPrivate::FunctorCall<QtPrivate::IndexesList<>, QtPrivate::List<>, void, KWin::ScreenCastStream::ScreenCastStream(KWin::ScreenCastSource*, QObject*)::<lambda()> >::call(struct {...} &, void **) (f=..., arg=0x7fffc9620040) at /usr/include/qt5/QtCore/qobjectdefs_impl.h:146
#5  0x0000000000498513 in QtPrivate::Functor<KWin::ScreenCastStream::ScreenCastStream(KWin::ScreenCastSource*, QObject*)::<lambda()>, 0>::call<QtPrivate::List<>, void>(struct {...} &, void *, void **) (f=..., arg=0x7fffc9620040) at /usr/include/qt5/QtCore/qobjectdefs_impl.h:256
#6  0x0000000000498458 in QtPrivate::QFunctorSlotObject<KWin::ScreenCastStream::ScreenCastStream(KWin::ScreenCastSource*, QObject*)::<lambda()>, 0, QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase *, QObject *, void **, bool *) (which=1, this_=0x2ebc400, r=0x3325120, a=0x7fffc9620040, ret=0x0) at /usr/include/qt5/QtCore/qobjectdefs_impl.h:443
#7  0x00007f344f034651 in QtPrivate::QSlotObjectBase::call(QObject*, void**) (a=0x7fffc9620040, r=<optimized out>, this=0x2ebc400)
    at ../../include/QtCore/../../src/corelib/kernel/qobjectdefs_impl.h:398
#8  doActivate<false>(QObject*, int, void**) (sender=0x3325310, signal_index=3, argv=0x7fffc9620040) at kernel/qobject.cpp:3923
#9  0x00007f344f02f4d7 in QMetaObject::activate(QObject*, QMetaObject const*, int, void**)
    (sender=<optimized out>, m=m@entry=0x7f344f2b2560 <QTimer::staticMetaObject>, local_signal_index=local_signal_index@entry=0, argv=argv@entry=0x7fffc9620040) at kernel/qobject.cpp:3983
#10 0x00007f344f037bcd in QTimer::timeout(QTimer::QPrivateSignal) (this=<optimized out>, _t1=...) at .moc/moc_qtimer.cpp:205
#11 0x00007f344f02b6bb in QObject::event(QEvent*) (this=0x3325310, e=0x7fffc96201b0) at kernel/qobject.cpp:1369
#12 0x00007f344d95ab75 in QApplicationPrivate::notify_helper(QObject*, QEvent*) (this=<optimized out>, receiver=0x3325310, e=0x7fffc96201b0) at kernel/qapplication.cpp:3640
#13 0x00007f344efffd48 in QCoreApplication::notifyInternal2(QObject*, QEvent*) (receiver=0x3325310, event=0x7fffc96201b0) at kernel/qcoreapplication.cpp:1064
#14 0x00007f344effff62 in QCoreApplication::sendEvent(QObject*, QEvent*) (receiver=<optimized out>, event=<optimized out>) at kernel/qcoreapplication.cpp:1462
#15 0x00007f344f051c6b in QTimerInfoList::activateTimers() (this=this@entry=0x17a4a68) at kernel/qtimerinfo_unix.cpp:643
#16 0x00007f344f04f020 in QEventDispatcherUNIXPrivate::activateTimers() (this=this@entry=0x17a49e0) at kernel/qeventdispatcher_unix.cpp:249
#17 0x00007f344f04fe70 in QEventDispatcherUNIX::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (this=<optimized out>, flags=...) at kernel/qeventdispatcher_unix.cpp:516
#18 0x00000000005a4d92 in QUnixEventDispatcherQPA::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
#19 0x00007f344effe70b in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (this=this@entry=0x7fffc9620350, flags=..., flags@entry=...)
    at ../../include/QtCore/../../src/corelib/global/qflags.h:69
#20 0x00007f344f00699b in QCoreApplication::exec() () at ../../include/QtCore/../../src/corelib/global/qflags.h:121
#21 0x00000000004a3154 in main(int, char**) (argc=14, argv=0x7fffc9620e18) at /home/carl/kde/src/kwin/src/main_wayland.cpp:628