A exclusive global mouse grabber can be dangerous and causing crash

In function QQuickDeliveryAgentPrivate::onGrabChanged(), the following call:

auto grabberItem = static_cast<QQuickItem *>(grabber);

can cause crash when one tries to have a global exclusive mouse grabber, i.e.., with setExclusivePointGrabber. The reason is that grabber can be a QObject here and static_cast<QQuickItem *> cannot process it correctly.