There's a race condition that may cause a client crash whenever a
video-provider's unmount procedure is run, but its signalling websocket
is undefined. The WS's callback handlers are re-assigned without
checking for the socket's availability, causing an unhandled TypeError.
The WS may be undefined in a couple of scenarios, e.g.: unmouting before
the socket was successfully set up, unmounting while a reconnect is in
place etc.
Check whether the socket exists before accessing it in video-provider's
componentWillUnmount routine.
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