in `block in combine_namespace_routes': undefined method `reject' for nil:NilClass (NoMethodError)

This issue looks similar to the issue fixed in #508.

~/workspace/ruby/test_grape_swagger hanfei@hanfei-rmbp
0:59:43 ❯ cat Gemfile
source 'https://rubygems.org'

gem 'grape', '~> 0.19'
gem 'grape-swagger', '~> 0.27'

~/workspace/ruby/test_grape_swagger hanfei@hanfei-rmbp
0:59:58 ❯ cat api.rb
require 'grape'
require 'grape-swagger'

module TheApi
  class ParentLessNamespaceApi < Grape::API
    route_param :animal do
      route_param :breed do
        resource :queues do
          route_param :queue_id do
            resource :reservations do
              desc 'Lists all reservations specific type of animal of specific breed in specific queue'
              get do
                { bla: 'Bla Black' }
              end
            end
          end
        end
      end
    end
  end
end

class ParentLessApi < Grape::API
  prefix :api
  mount TheApi::ParentLessNamespaceApi
  add_swagger_documentation version: 'v1'
end

~/workspace/ruby/test_grape_swagger hanfei@hanfei-rmbp
1:00:06 ❯ ruby -v
ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-darwin16]

~/workspace/ruby/test_grape_swagger hanfei@hanfei-rmbp
1:00:32 ❯ ruby api.rb
/Users/hanfei/.rvm/gems/ruby-2.4.0/gems/grape-swagger-0.27.2/lib/grape-swagger.rb:100:in `block in combine_namespace_routes': undefined method `reject' for nil:NilClass (NoMethodError)
        from /Users/hanfei/.rvm/gems/ruby-2.4.0/gems/grape-swagger-0.27.2/lib/grape-swagger.rb:95:in `each'
        from /Users/hanfei/.rvm/gems/ruby-2.4.0/gems/grape-swagger-0.27.2/lib/grape-swagger.rb:95:in `combine_namespace_routes'
        from /Users/hanfei/.rvm/gems/ruby-2.4.0/gems/grape-swagger-0.27.2/lib/grape-swagger.rb:49:in `add_swagger_documentation'
        from api.rb:26:in `<class:ParentLessApi>'
        from api.rb:23:in `<main>'

Reproduce the failure with the api.rb example, then inspect combine_namespace_routes in grape-swagger.rb around lines 95-100 and compare the related fix in issue #508. Done means the nested mounted routes no longer raise NoMethodError and add_swagger_documentation completes successfully.