Fixing various edge-cases when two gUM error dialogs might be shown and other possible bugs

modules/UI/UI.js

 
 let followMeHandler;
 
+let deviceErrorDialog;
+
 const TrackErrors = JitsiMeetJS.errors.track;
 
 const JITSI_TRACK_ERROR_TO_MESSAGE_KEY_MAP = {
 
     message = `${message}${doNotShowWarningAgainSection}`;
 
-    messageHandler.openDialog(
+    // To make sure we don't have multiple error dialogs open at the same time,
+    // we will just close the previous one if we are going to show a new one.
+    deviceErrorDialog && deviceErrorDialog.close();
+
+    deviceErrorDialog = messageHandler.openDialog(
         titleMsg,
         message,
         false,

modules/devices/mediaDeviceHelper.js

             return createLocalTracks(
                     ['audio', 'video'], cameraDeviceId, micDeviceId)
                     // If we fail to do this, try to create them separately.
-                    .catch(() => Promise.all(
-                        [createAudioTrack(false), createVideoTrack(false)]))
-                    .then((audioTracks, videoTracks) => {
+                    .catch(() => Promise.all([
+                        createAudioTrack(false).then(([stream]) => stream),
+                        createVideoTrack(false).then(([stream]) => stream)
+                    ]))
+                    .then(tracks => {
                         if (audioTrackError || videoTrackError) {
                             APP.UI.showDeviceErrorDialog(
                                 audioTrackError, videoTrackError);
                         }
 
-                        return (audioTracks || []).concat(videoTracks || []);
+                        return tracks.filter(t => typeof t !== 'undefined');
                     });
         } else if (videoRequested && !audioRequested) {
             return createVideoTrack();