Fails to import into Swagger Editor - extra comma in generated JSON file

[belgoros]

I followed the README guidelines and created a JSON file after copy-pasting JSON response when hitting http://localhost:3000/apidocs URL.
Swagger Editor fails to accept it saying:

Here is ApidocsController:

class ApidocsController < ApplicationController
  skip_before_action :authorize_request
  
  include Swagger::Blocks
  
  swagger_root do
    key :swagger, '2.0'
    info do
      key :version, '1.0.0'
      key :title, 'Example Project API'
      key :description, 'A Swagger Project API that uses a bla-bla-bla project features ' \
                        'in commliance with the swagger-2.0 specification'
      contact do
        key :name, 'Fancy API Team'
      end
      license do
        key :name, 'MIT'
      end
    end
    
    tag do
      key :name, 'fancy_project'
      key :description, 'Example Project API operations'
      externalDocs do
        key :description, 'Find more info here'
        key :url, 'https://swagger.io'
      end
    end
    key :host, 'https://api-eu.swagger/example-project'
    key :basePath, '/v1'
    key :consumes, ['application/json']
    key :produces, ['application/json']
  end
  
  # A list of all classes that have swagger_* declarations.
    SWAGGERED_CLASSES = [
      V1::LanguagesController,
      Language,
      ErrorModel,
      self,
    ].freeze

    def index
      render json: Swagger::Blocks.build_root_json(SWAGGERED_CLASSES)
    end
end

Here is V1::LanguagesController code:

module V1
  class LanguagesController < ApplicationController
    include Swagger::Blocks

    before_action :find_language, only: :show

    swagger_path '/languages' do
      operation :get do
        key :summary, 'All Language tags'
        key :description, 'Returns all language ISO tags from the system that the user has access to'
        key :produces, [
          'application/json'
        ]
        key :tags, [
          'language'
        ]
        response 200 do
          key :description, 'Languages response'
          schema do
            key :type, :array
            items do
              key :'$ref', :Language
            end
          end
        end

        response :default do
          key :description, 'unexpected error'
          schema do
            key :'$ref', :ErrorModel
          end
        end
      end
    end

    def index
      @languages = Language.all
      json_response @languages, :ok
    end
...

What am I missing ?
Rails 5.2. (API-mode)
ruby 2.5.0

You cat find swagger.json content in this gist.