Workaround ChangeListener getting called on semi-deleted objects

Without this change, the emit in itemVisibilityChanged would trigger
QFATAL : tst_controls::Basic::Dialog::test_layout()
ASSERT failure in QQuickDialog: "Called object is not of the correct type
(class destructor may have already run)"
in tst_basic, as the object was already semi-destroyed.

Task-number: QTBUG-98790
Change-Id: I112df9da0c07a1f9c14b8d557c1c1c19405d3180
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

src/quicktemplates2/qquickpage.cpp

@@ -164,6 +164,8 @@ void QQuickPagePrivate::itemVisibilityChanged(QQuickItem *item)
 {
     Q_Q(QQuickPage);
     QQuickPanePrivate::itemVisibilityChanged(item);
+    if (QQmlData::wasDeleted(q))
+        return;
     if (item == header) {
         QBoolBlocker signalGuard(emittingImplicitSizeChangedSignals);
         emit q->implicitHeaderWidthChanged();