No Ransack::Search object was provided to search_form_for! when using ActiveAdmin

routes.rb

MyAppName::Application.routes.draw do
  # ...  
  namespace :admin do
    resources :sales do
      resources :consignment_items do
        collection { post :search_items, to: 'sales#search_consignment_items' }
      end
    end
  end
  # ...
end

sales.rb

# in app/admin/sales.rb
ActiveAdmin.register Sale  do
  # ...
  # Available product filter panel
  sidebar :available_product_filters, only: [:new, :edit] do
      search_form_for @search, url: 'search_consignment_items_admin_sale', remote: true do |f|
          f.condition_fields do |c|
              render "condition_fields", f: c
          end
          f.actions('submit')
      end
  end

  member_action :search_consignment_items, method: :post do
    @search = ConsignmentItem.ransack(params[:search])
    @consignment_items = @search.result
    @search.build_condition if @search.conditions.empty?
    @search.build_sort if @search.sorts.empty?
  end
end