ref(conference.js): remove global promise

Get rid of global APP.conference.screenSharingPromise.

conference.js

     },
 
     videoSwitchInProgress: false,
+
+    /**
+     * Toggles between screensharing and camera video.
+     * @param {boolean} [shareScreen]
+     * @return {Promise.<T>}
+     */
     toggleScreenSharing(shareScreen = !this.isSharingScreen) {
         if (this.videoSwitchInProgress) {
-            logger.warn("Switch in progress.");
-            return;
+            return Promise.reject('Switch in progress.');
         }
         if (!this.isDesktopSharingEnabled) {
-            logger.warn("Cannot toggle screen sharing: not supported.");
-            return;
+            return Promise.reject(
+                'Cannot toggle screen sharing: not supported.');
         }
 
         if (this.isAudioOnly()) {
             this._displayAudioOnlyTooltip('screenShare');
-            return;
+
+            return Promise.reject('No screensharing in audio only mode');
         }
 
         this.videoSwitchInProgress = true;
         let externalInstallation = false;
 
         if (shareScreen) {
-            this.screenSharingPromise = createLocalTracks({
+            return createLocalTracks({
                 devices: ['desktop'],
                 desktopSharingExtensionExternalInstallation: {
                     interval: 500,
             });
         } else {
             APP.remoteControl.receiver.stop();
-            this.screenSharingPromise = createLocalTracks(
+            return createLocalTracks(
                 { devices: ['video'] })
             .then(
                 ([stream]) => this.useVideoStream(stream)

modules/remotecontrol/Receiver.js

                 action: PERMISSIONS_ACTIONS.grant
             });
         } else {
-            APP.conference.toggleScreenSharing();
-            APP.conference.screenSharingPromise.then(() => {
-                if (APP.conference.isSharingScreen) {
-                    this.sendRemoteControlEvent(userId, {
-                        type: EVENT_TYPES.permissions,
-                        action: PERMISSIONS_ACTIONS.grant
-                    });
-                } else {
+            APP.conference.toggleScreenSharing()
+                .then(() => {
+                    if (APP.conference.isSharingScreen) {
+                        this.sendRemoteControlEvent(userId, {
+                            type: EVENT_TYPES.permissions,
+                            action: PERMISSIONS_ACTIONS.grant
+                        });
+                    } else {
+                        this.sendRemoteControlEvent(userId, {
+                            type: EVENT_TYPES.permissions,
+                            action: PERMISSIONS_ACTIONS.error
+                        });
+                    }
+                })
+                .catch(() => {
                     this.sendRemoteControlEvent(userId, {
                         type: EVENT_TYPES.permissions,
                         action: PERMISSIONS_ACTIONS.error
                     });
-                }
-            }).catch(() => {
-                this.sendRemoteControlEvent(userId, {
-                    type: EVENT_TYPES.permissions,
-                    action: PERMISSIONS_ACTIONS.error
                 });
-            });
         }
     }