From the declaration of CreateDataChannel at various layers (webrtc, content, blink), it is clear that it returns a null pointer if creation fails.

The spec says that it should return various error codes for various conditions, including TypeError and StateError.

http://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-createdatachannel

Either all checking must be done before we create it (leaving other errors as InternalError), or we must pass back an error code from the lower layers.