What is your nodebb git hash?
On latest versions topics.loadMore doesn't return mainPost anymore.
If you are running latest master update to get these changes https://github.com/NodeBB/NodeBB/commit/906dc5675e5e1e96a7ff693f07f20987cd08c87a.
Then the main post will be the first item in the posts array when you call topics.loadMore. Then you can use d.posts.content instead of d.mainPost.content