fix(conference.js): crash on undefined this.localPresenterVideo

...if camera device is changed in the settings while screen sharing.

conference.js

 
                     // dispose the existing presenter track and create a new
                     // camera track.
-                    this.localPresenterVideo.dispose();
+                    this.localPresenterVideo && this.localPresenterVideo.dispose();
                     this.localPresenterVideo = null;
 
                     return this._createPresenterStreamEffect(height, cameraDeviceId)
                     const { height } = this.localVideo.track.getSettings();
 
                     this._updateVideoDeviceId();
-                    this.localPresenterVideo.dispose();
+                    this.localPresenterVideo && this.localPresenterVideo.dispose();
                     this.localPresenterVideo = null;
                     this._createPresenterStreamEffect(height, cameraDeviceId);