This prevents a crash when the DeviceManager tries to bring up a UI during
its destruction.

This happens in the same situation as described in QTCREATORBUG-11712,
which actually describes the same problem in BlackBerryConfigurationManager.

Change-Id: I06f134ffa282a281c81fd022c5a2f52fb5dddb56
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>