Exception when calling WebRTC Class, while connecting to a peer to peer Connection

Question

I am connecting with the signaling server using an AppRTC Peer to peer connection. I use WebRTC Framework. All file & classes are working fine and the setup seems correct.

But when I call this webRTC class in my videoViewController, the application crashes with this error message:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Swift._NSContiguousString key]: unrecognized selector sent to instance 0x2805b5d00'

which indicates that some value is nil, or not declared. But I am not able to find which value is nil or not declared in AppRTC Peer to peer connection class.

Any help would be appreciated to find why.

Swift language:

fileprivate let audioCallConstraint = RTCMediaConstraints(mandatoryConstraints: ["OfferToReceiveAudio" : "true"], optionalConstraints: nil)

fileprivate let videoCallConstraint = RTCMediaConstraints(mandatoryConstraints: ["OfferToReceiveAudio" : "true", "OfferToReceiveVideo": "true"], optionalConstraints: nil)

Message:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Swift._NSContiguousString key]: unrecognized selector sent to instance 0x2805b5d00'