fix(desktop_sharing): if remote control is disabled

conference.js

     _turnScreenSharingOff(didHaveVideo, wasVideoMuted) {
         this._untoggleScreenSharing = null;
         this.videoSwitchInProgress = true;
-        APP.remoteControl.receiver.stop();
+        const { receiver } = APP.remoteControl;
+
+        if (receiver) {
+            receiver.stop();
+        }
+
         let promise = null;
 
         if (didHaveVideo) {