Adding a new Category from the Pages admin view results in an error

[morgant]

Expected behavior

On the Pages admin view (e.g. /admin/sites/1/pages), clicking the "Edit" button below the categories list, entering a new (unused) Category label, and clicking the '+' button should create the new Category and dynamically show it in the categories list.

Actual behavior

The new Category is created, but the browser will load a blank page (in production) or generate the following error (in development):

ActionController::UnknownFormat in Comfy::Admin::Cms::CategoriesController#create
Comfy::Admin::Cms::CategoriesController#create is missing a template for this request format and variant. request.formats: ["text/html"] request.variant: []

Steps to reproduce

1. Load the Pages admin view (e.g. /admin/sites/1/pages)
2. Click the 'Edit' button below the categories list in the sidebar
3. Enter a new, unused Category label/name in the "Add Category" text field
4. Click the '+' button

System configuration

CMS version: 2.0.18

Ruby version: 2.7.4

Rails version: 6.0.1