fix(RTC): channel not open while trying to send

Remove redundant _channelOpen flag from RTC and rely on the channel's isOpen() getter instead.
This fixes an error throw when the _channelOpen flag was not updated while the channel's been
closed by the bridge.

modules/RTC/RTC.js

         // @type {BridgeChannel}
         this._channel = null;
 
-        // A flag whether we had received that the channel had opened we can
-        // get this flag out of sync if for some reason channel got closed
-        // from server, a desired behaviour so we can see errors when this
-        // happen.
-        // @private
-        // @type {boolean}
-        this._channelOpen = false;
-
         /**
          * The value specified to the last invocation of setLastN before the
          * channel completed opening. If non-null, the value will be sent
             peerconnection, wsUrl, this.eventEmitter, this._senderVideoConstraintsChanged.bind(this));
 
         this._channelOpenListener = () => {
-            // Mark that channel as opened.
-            this._channelOpen = true;
-
             // When the channel becomes available, tell the bridge about
             // video selections so that it can do adaptive simulcast,
             // we want the notification to trigger even if userJid
             }
 
             this._channel = null;
-            this._channelOpen = false;
         }
     }
 
     setReceiverVideoConstraint(maxFrameHeight) {
         this._maxFrameHeight = maxFrameHeight;
 
-        if (this._channel && this._channelOpen) {
+        if (this._channel && this._channel.isOpen()) {
             this._channel.sendReceiverVideoConstraintMessage(maxFrameHeight);
         }
     }
     selectEndpoints(ids) {
         this._selectedEndpoints = ids;
 
-        if (this._channel && this._channelOpen) {
+        if (this._channel && this._channel.isOpen()) {
             this._channel.sendSelectedEndpointsMessage(ids);
         }
     }
     pinEndpoint(id) {
         // Cache the value if channel is missing, till we open it.
         this._pinnedEndpoint = id;
-        if (this._channel && this._channelOpen) {
+        if (this._channel && this._channel.isOpen()) {
             this._channel.sendPinnedEndpointMessage(id);
         }
     }
     closeBridgeChannel() {
         if (this._channel) {
             this._channel.close();
-            this._channelOpen = false;
+            this._channel = null;
 
             this.removeListener(RTCEvents.LASTN_ENDPOINT_CHANGED,
                 this._lastNChangeListener);
     setLastN(value) {
         if (this._lastN !== value) {
             this._lastN = value;
-            if (this._channel && this._channelOpen) {
+            if (this._channel && this._channel.isOpen()) {
                 this._channel.sendSetLastNMessage(value);
             }
             this.eventEmitter.emit(RTCEvents.LASTN_VALUE_CHANGED, value);