QAccessibleObject::childAt crash when child is NULL

show an embed widget (from another process,use QWindow::from and QWidget::createWindowContainer) crashed in qaccessibleobject.cpp line127.

// code placeholder
for (int i = 0; i < childCount(); ++i) {
QAccessibleInterface *childIface = child(i);
Q_ASSERT(childIface);
if (childIface->isValid() && childIface->rect().contains(x,y))
return childIface;
}
return 0;

As Qt assistant mention "Returns 0 when asking for an invalid child (e.g. when the child became invalid in the meantime).",  here child return NULL.