Cannot see Schema visualisation on the playground

[circap-ppb]

After building a really basic graphql application with queries, I was trying to check the schema by using the Playground (/gui).
I could see that if the query return POJO created by me (for example User.java), the Playground doesn't allow me to check the schema.

@GraphQLApi
public class SpqrTest {

    @GraphQLQuery
    public User getObject() {
        User user = new User();
        user.setName("name");
        return user;
    }
}

public class User {
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    private String name;
}

by going into http://localhost:8080/gui
and them select schema
it appears an empty page and by inspecting it, I could see this:

react-dom.production.min.js:209 SyntaxError: Syntax Error: Unexpected Name "name" (5:3)
  3 | 
  4 | 
> 5 |   name: String
    |   ^
  6 | }
  7 | 
  8 | 
    at t.start.line (parser-graphql.js:1)
    at Object.parse (parser-graphql.js:1)
    at Object.parse (standalone.js:13721)
    at Hs (standalone.js:17145)
    at Ws (standalone.js:17381)
    at formatWithCursor (standalone.js:17397)
    at standalone.js:32917
    at Object.format (standalone.js:32926)
    at Object.t.prettify (utils.ts:22)
    at Object.l [as getSDL] (createSDL.ts:94)

If the queries created by me return String or Integer, I can see the schema on the Playground.