Null check userId on member. Fixes #5.

src/Avatar.js

@@ -27,7 +27,10 @@ module.exports = {
             resizeMethod
         );
         if (!url) {
-            url = this.defaultAvatarUrlForString(member.userId);
+            // member can be null here currently since on invites, the JS SDK
+            // does not have enough info to build a RoomMember object for
+            // the inviter.
+            url = this.defaultAvatarUrlForString(member ? member.userId : '');
         }
         return url;
     },