Fix crash when setting Qt::WA_StaticContents on a transient child

Bail out of QWidgetBackingStore::updateLists() when a
top level transient child is encountered, preventing from
adding children of other top level windows to its list.

Task-number: QTBUG-70085
Change-Id: Ibd748ab959af9bfe7fe0622494936b0c109ab25a
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/widgets/kernel/qwidgetbackingstore.cpp

@@ -760,7 +760,7 @@ void QWidgetBackingStore::updateLists(QWidget *cur)
     QList<QObject*> children = cur->children();
     for (int i = 0; i < children.size(); ++i) {
         QWidget *child = qobject_cast<QWidget*>(children.at(i));
-        if (!child)
+        if (!child || child->isWindow())
             continue;
 
         updateLists(child);