Illegal memory access during destruction of QQuickShaderEffect

QQuickShaderEffectPrivate accesses QQuickShaderEffect after it has been destroyed.

It is not possible to build desktop with -sanitize undefined, but it is possible for the webassembly build, which gives this calllstack:

 
pptestGrabWindow.js:1157 /home/evenan/source/repos/6.8.0/qt6/qtdeclarative/src/quick/items/qquickshadereffect_p_p.h:34:5: runtime error: downcast of address 0x054ea658 which does not point to an object of type 'QQuickShaderEffect'
put_char @ apptestGrabWindow.js:1157
write @ apptestGrabWindow.js:1109
write @ apptestGrabWindow.js:2471
doWritev @ apptestGrabWindow.js:13118
_fd_write @ apptestGrabWindow.js:13135
$__sanitizer::internal_write(int, void const*, unsigned long) @ apptestGrabWindow.wasm:0xbec4240
$__sanitizer::ReportFile::Write(char const*, unsigned long) @ apptestGrabWindow.wasm:0xbec512d
$__sanitizer::RawWrite(char const*) @ apptestGrabWindow.wasm:0xbebe929
$__sanitizer::SharedPrintfCodeNoBuffer(bool, char*, int, char const*, void*) @ apptestGrabWindow.wasm:0xbec6677
$__sanitizer::SharedPrintfCode(bool, char const*, void*) @ apptestGrabWindow.wasm:0xbec6302
$__sanitizer::Printf(char const*, ...) @ apptestGrabWindow.wasm:0xbec6224
$__ubsan::Diag::~Diag() @ apptestGrabWindow.wasm:0xbeb0a4b
$HandleDynamicTypeCacheMiss(__ubsan::DynamicTypeCacheMissData*, unsigned long, unsigned long, __ubsan::ReportOptions) @ apptestGrabWindow.wasm:0xbeb847f
$__ubsan_handle_dynamic_type_cache_miss @ apptestGrabWindow.wasm:0xbeb8094
$QQuickShaderEffectPrivate::q_func() @ apptestGrabWindow.wasm:0x20fff71
$QQuickShaderEffectPrivate::disconnectSignals(QQuickShaderEffectPrivate::Shader) @ apptestGrabWindow.wasm:0x211a5f4
$QQuickShaderEffectPrivate::~QQuickShaderEffectPrivate() @ apptestGrabWindow.wasm:0x2119e48
$QQuickShaderEffectPrivate::~QQuickShaderEffectPrivate().1 @ apptestGrabWindow.wasm:0x211eb27
$QScopedPointerDeleter<QObjectData>::cleanup(QObjectData*) @ apptestGrabWindow.wasm:0x712e05
$QScopedPointer<QObjectData, QScopedPointerDeleter<QObjectData>>::~QScopedPointer() @ apptestGrabWindow.wasm:0x70f533
$QObject::~QObject() @ apptestGrabWindow.wasm:0x7172a1
$QQuickItem::~QQuickItem() @ apptestGrabWindow.wasm:0x2af14b6
$QQuickShaderEffect::~QQuickShaderEffect() @ apptestGrabWindow.wasm:0x20fe462
$QQmlPrivate::QQmlElement<QQuickShaderEffect>::~QQmlElement() @ apptestGrabWindow.wasm:0x41fe043
$QQmlPrivate::QQmlElement<QQuickShaderEffect>::~QQmlElement().1 @ apptestGrabWindow.wasm:0x41fe15e
$QObject::event(QEvent*) @ apptestGrabWindow.wasm:0x724af4
$QQuickItem::event(QEvent*) @ apptestGrabWindow.wasm:0x2bb40a3
$QQuickShaderEffect::event(QEvent*) @ apptestGrabWindow.wasm:0x210a1b5
$QCoreApplicationPrivate::notify_helper(QObject*, QEvent*) @ apptestGrabWindow.wasm:0x48d6ee
$doNotify(QObject*, QEvent*) @ apptestGrabWindow.wasm:0x48caad
$QCoreApplication::notify(QObject*, QEvent*) @ apptestGrabWindow.wasm:0x48d9e9
$QGuiApplication::notify(QObject*, QEvent*) @ apptestGrabWindow.wasm:0x5fad2d5
$dynCall_iiii @ apptestGrabWindow.wasm:0xbefc682
ret.<computed> @ apptestGrabWindow.js:6228
invoke_iiii @ apptestGrabWindow.js:14532
$QCoreApplication::notifyInternal2(QObject*, QEvent*) @ apptestGrabWindow.wasm:0x48c660
$QCoreApplication::sendEvent(QObject*, QEvent*) @ apptestGrabWindow.wasm:0x49a803
$dynCall_iii @ apptestGrabWindow.wasm:0xbefc5b4
ret.<computed> @ apptestGrabWindow.js:6228
invoke_iii @ apptestGrabWindow.js:14521
$QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) @ apptestGrabWindow.wasm:0x498d23
$QCoreApplication::sendPostedEvents(QObject*, int) @ apptestGrabWindow.wasm:0x4934c0
$QEventDispatcherWasm::processPostedEvents() @ apptestGrabWindow.wasm:0x1623c30
$QWasmEventDispatcher::processPostedEvents() @ apptestGrabWindow.wasm:0x47f5bf6
$QEventDispatcherWasm::callProcessPostedEvents(void*) @ apptestGrabWindow.wasm:0x1623b0f
$QEventDispatcherWasm::wakeUp()::$_0::operator()() const @ apptestGrabWindow.wasm:0x166721f
 
As to the consequences they are unknown except that
1) It is probably U.B.
2) It shows up when debugging
 
Seen when debugging QTBUG-124152
which can be used as a reproducer