#include <QGuiApplication>
#include <QQuickWindow>
#include <QTimer>

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQuickWindow view1;
    auto view2 = new QQuickWindow(&view1);

    view1.show();
    view2->show();

    QTimer::singleShot(1000, &app, &QGuiApplication::quit);

    return app.exec();
} 

(gdb) bt

#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44

#1  0x00007ffff3a98a13 in __pthread_kill_internal (threadid=<optimized out>, signo=6) at pthread_kill.c:89

#2  0x00007ffff3a3e410 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26

#3  0x00007ffff3a2557a in __GI_abort () at abort.c:77

#4  0x00007ffff42c75de in qAbort () at /pub_data/sources/qt/qt6/qtbase/src/corelib/global/qassert.cpp:46

#5  0x00007ffff435c929 in qt_maybe_message_fatal<QString&> (msgType=<optimized out>, context=..., message=...) at /pub_data/sources/qt/qt6/qtbase/src/corelib/global/qlogging.cpp:2168

#6  qt_message(QtMsgType, const QMessageLogContext &, const char *, typedef __va_list_tag __va_list_tag *) (msgType=msgType@entry=QtFatalMsg, context=..., msg=msg@entry=0x7ffff4795f18 "ASSERT failure in %s: \"%s\", file %s, line %d", ap=ap@entry=0x7fffffffc228)

    at /pub_data/sources/qt/qt6/qtbase/src/corelib/global/qlogging.cpp:412

#7  0x00007ffff42c98fb in QMessageLogger::fatal (this=this@entry=0x7fffffffc460, msg=msg@entry=0x7ffff4795f18 "ASSERT failure in %s: \"%s\", file %s, line %d") at /pub_data/sources/qt/qt6/qtbase/src/corelib/global/qlogging.cpp:901

#8  0x00007ffff42c769f in qt_assert_x (where=<optimized out>, what=what@entry=0x7ffff6ed3c30 "Called object is not of the correct type (class destructor may have already run)", file=file@entry=0x7ffff6ed3be8 "/pub_data/sources/qt/qt6/qtbase/src/corelib/kernel/qobjectdefs_impl.h", line=line@entry=105)

    at /pub_data/sources/qt/qt6/qtbase/src/corelib/global/qassert.cpp:119

#9  0x00007ffff6b2cf64 in QtPrivate::assertObjectType<QQuickWindow> (o=<optimized out>) at /pub_data/sources/qt/qt6/qtbase/src/corelib/kernel/qobjectdefs_impl.h:94

#10 QtPrivate::assertObjectType<QQuickWindow> (o=<optimized out>) at /pub_data/sources/qt/qt6/qtbase/src/corelib/kernel/qobjectdefs_impl.h:94

#11 QtPrivate::FunctorCall<std::integer_sequence<unsigned long>, QtPrivate::List<>, void, void (QQuickWindow::*)()>::call(void (QQuickWindow::*)(), QQuickWindow*, void**) (f=<optimized out>, o=<optimized out>, arg=<optimized out>) at /pub_data/sources/qt/qt6/qtbase/src/corelib/kernel/qobjectdefs_impl.h:125

#12 QtPrivate::FunctionPointer<void (QQuickWindow::*)()>::call<QtPrivate::List<>, void>(void (QQuickWindow::*)(), QQuickWindow*, void**) (f=<optimized out>, o=<optimized out>, arg=<optimized out>) at /pub_data/sources/qt/qt6/qtbase/src/corelib/kernel/qobjectdefs_impl.h:174

#13 QtPrivate::QCallableObject<void (QQuickWindow::*)(), QtPrivate::List<>, void>::impl(int, QtPrivate::QSlotObjectBase*, QObject*, void**, bool*) (which=<optimized out>, this_=<optimized out>, r=<optimized out>, a=<optimized out>, ret=<optimized out>)

    at /pub_data/sources/qt/qt6/qtbase/src/corelib/kernel/qobjectdefs_impl.h:545

#14 0x00007ffff4468ee5 in QtPrivate::QSlotObjectBase::call (this=0x55555564ae20, r=<optimized out>, a=0x7fffffffc540) at /pub_data/sources/qt/qt6/qtbase/src/corelib/kernel/qobjectdefs_impl.h:461

#15 doActivate<false> (sender=0x55555564e400, signal_index=4, argv=0x7fffffffc540) at /pub_data/sources/qt/qt6/qtbase/src/corelib/kernel/qobject.cpp:4314

#16 0x00007ffff6b6af8a in QSGSoftwareRenderLoop::windowDestroyed (this=0x5555556490b0, window=<optimized out>) at /pub_data/sources/qt/qt6/qtdeclarative/src/quick/scenegraph/adaptations/software/qsgsoftwarerenderloop.cpp:70

#17 0x00007ffff6b37e0c in QQuickWindow::~QQuickWindow (this=0x55555564b330) at /pub_data/sources/qt/qt6/qtdeclarative/src/quick/items/qquickwindow.cpp:1185

#18 0x00007ffff6b37e62 in QQuickWindow::~QQuickWindow (this=0x55555564b330) at /pub_data/sources/qt/qt6/qtdeclarative/src/quick/items/qquickwindow.cpp:1216

#19 0x00007ffff5ed8255 in qDeleteAll<QList<QWindow*>::const_iterator> (begin=..., end=...) at /pub_data/sources/qt/qt6/qtbase/src/corelib/tools/qalgorithms.h:24

#20 qDeleteAll<QList<QWindow*> > (c=...) at /pub_data/sources/qt/qt6/qtbase/src/corelib/tools/qalgorithms.h:32

#21 QWindow::~QWindow (this=this@entry=0x7fffffffc710) at /pub_data/sources/qt/qt6/qtbase/src/gui/kernel/qwindow.cpp:197

#22 0x00007ffff6b37be2 in QQuickWindow::~QQuickWindow (this=this@entry=0x7fffffffc710) at /pub_data/sources/qt/qt6/qtdeclarative/src/quick/items/qquickwindow.cpp:1216

#23 0x00005555556133f3 in main (argc=<optimized out>, argv=<optimized out>) at main.cpp:19