QObject::destroyed() is emitted too late for proper unregistering from QOpenUrlHandlerRegistry

When a URL handler is destroyed, it is removed from the registry only by way of the destroyed() signal. Even if QTBUG-100778 is fixed, there's still a window where the handler's salient members have already been destroyed, and thus it cannot act as a handler anymore, but it hasn't been removed from the registry, yet, and thus can still be invoked by a call to openUrl().