fix: TypeError: Cannot read property 'isAudioTrack' of undefined

When there's no jitsiTrack set on the base/tracks entry it means
a track is being created (get user media is in progress).

react/features/base/tracks/functions.js

         mediaType,
         participantId) {
     return tracks.find(
-        t => t.participantId === participantId && t.mediaType === mediaType
+        t => Boolean(t.jitsiTrack) && t.participantId === participantId && t.mediaType === mediaType
     );
 }
 

react/features/prejoin/middleware.js

             await dispatch(replaceLocalTrack(localVideoTrack.jitsiTrack, null));
         }
 
-        const jitsiTracks = getState()['features/base/tracks'].map(t => t.jitsiTrack);
+        const jitsiTracks = getState()['features/base/tracks']
+            .map(t => t.jitsiTrack)
+            .filter(t => Boolean(t)); // Filter out GUM in progress tracks...
 
         dispatch(setPrejoinPageVisibility(false));
         APP.conference.prejoinStart(jitsiTracks);