Fix 'user_id' of undefined JS error (#6878)

webapp/components/post_view/post_body/index.js

@@ -16,7 +16,7 @@ function mapStateToProps(state, ownProps) {
     let parentPostUser;
     if (ownProps.post.root_id) {
         parentPost = getPost(state, ownProps.post.root_id);
-        parentPostUser = getUser(state, parentPost.user_id);
+        parentPostUser = parentPost ? getUser(state, parentPost.user_id) : null;
     }
 
     return {