fix: undefined participant in avatar call

react/features/external-api/middleware.js

 
         const result = next(action);
 
-        if (participant.loadableAvatarUrl !== loadableAvatarUrl) {
+        if (participant && (participant.loadableAvatarUrl !== loadableAvatarUrl)) {
             APP.API.notifyAvatarChanged(
                 id,
                 loadableAvatarUrl