Improves startup and fix connectivity when Media devices exist in the fi...

When the first group contains the device of type "MediaTransmitter" the json deserialization fails because it cannot parse the enum.
The fix is to add "MediaTransmitter" to DeviceTypes.

I also changed try { ....First(..) } to FirstOrDefault(...) to avoid the hit of exception handling and improve perf.

Thanks,

Oscar Calvo