Exception when disconnecting interaction (crashing the client)

[MatthiasKohl]

I have an issue that whenever a Vidyo interaction is disconnected, the client crashes.
This is due to the following:
The InteractionViewModel._participantCheckerTimer will still be running when the interaction disconnects.
Next time the timer times out, the VidyoServiceClient.GetParticipants function is called and will generate an exception in the integration service as well as in the client.
The function then returns null and then this line here will generate a NullReferenceException:
Context.Send(s => Participants.AddRange(participants, true), null);
Since the NullReferenceException is generated inside the synchronization context, Client/Desktop doesn't know what to do with it and crashes.
I quick-fixed that second exception by throwing an exception when VidyoServiceClient.GetParticipants returns null (which then gets handled normally), but that's really not the way to solve this.

Instead, the timer should be stopped inside a lock when the interaction disconnects and the ParticipantCheckerTimerOnElapsed function should have that same lock.

Also, the timer has 'AutoReset=true' but then gets stopped and started manually, that's a bit confusing.