From PR https://github.com/ManageIQ/manageiq/pull/13024: When creating a DialogFieldTagControl, the options were not being symbolized. When returning the content of a DialogField, it was then raising a 'NoMethodError' because category was returning nil since it is looking for a symbolized category_id, not a string.
Fixed. Verified in 5.8.0.13-rc2.20170502165848_0f98658