Here is a quick look to help figure out the issue before bed. 
displayName doesn’t exist. There is a name field instead.
The changes happen in this commit:
https://github.com/discourse/discourse/commit/c197daa04c3114184cbd3ec2f1cfade1b5701793
It seems to miss the getters in the context of user summary since the data is retrieved from /summary.json.
Maybe mapping top_categories with Category.create() or a basic specific class would be okay.