discourse/app/assets/javascripts/admin/routes/admin-site-settings-index.js

/**
  Handles when you click the Site Settings tab in admin, but haven't
  chosen a category. It will redirect to the first category.
**/
import DiscourseRoute from "discourse/routes/discourse";

export default DiscourseRoute.extend({
  beforeModel() {
    this.replaceWith(
      "adminSiteSettingsCategory",
      this.controllerFor("adminSiteSettings").get("visibleSiteSettings")[0]
        .nameKey
    );
  }
});