bindings[:object] / polymorphic_association return nil in nested form

[fmh]

a really strange issue in my use case :

class OrderItem < ActiveRecord::Base
  has_many :reports, as: :eventable, class_name: 'EventReport', dependent: :delete_all
end

class Event < ActiveRecord::Base
  belongs_to :eventable, polymorphic: true
end

 #undefined method `!!model_method!!!' for nil:NilClass
class EventReport < Event
  belongs_to :order_item
  rails_admin do  
   nested do
    field :a
    field :b
    field :c do 
      visible do 
        bindings[:object].order_item.checked #### order_item is null ???? undefined method `checked' for nil:NilClass
     end
  end
end

class OrderItem < ActiveRecord::Base
   has_many :reports, as: :eventable, class_name: 'EventReport', dependent: :delete_all
   rails_admin do
      nested do
          field :reports    # field c not rendered and get undefined method `checked' for nil:NilClass   
      end
  end
end

it work fine when access directly from event_report model, but not within nested model or nested form

app/models/event_report.rb:119:in `block (4 levels) in <class:EventReport>'
app/helpers/rails_admin/form_builder.rb:106:in `field_for'
app/helpers/rails_admin/form_builder.rb:90:in `block in input_for'
app/helpers/rails_admin/form_builder.rb:77:in `input_for'
app/helpers/rails_admin/form_builder.rb:64:in `block (2 levels) in field_wrapper_for'
app/helpers/rails_admin/form_builder.rb:62:in `block in field_wrapper_for'
app/helpers/rails_admin/form_builder.rb:60:in `field_wrapper_for'
app/helpers/rails_admin/form_builder.rb:28:in `block (2 levels) in fieldset_for'
app/helpers/rails_admin/form_builder.rb:28:in `collect'
app/helpers/rails_admin/form_builder.rb:28:in `block in fieldset_for'
app/helpers/rails_admin/form_builder.rb:24:in `fieldset_for'
app/helpers/rails_admin/form_builder.rb:16:in `block (2 levels) in generate'
app/helpers/rails_admin/form_builder.rb:15:in `collect'
app/helpers/rails_admin/form_builder.rb:15:in `block in generate'
app/helpers/rails_admin/form_builder.rb:178:in `without_field_error_proc_added_div'
app/helpers/rails_admin/form_builder.rb:7:in `generate'
app/views/rails_admin/main/_polyapp_nested_many.html.haml:9:in `block in _app_views_rails_admin_main__polyapp_nested_many_html_haml___2974600304240866387_70176537295020'
app/helpers/rails_admin/main_helper.rb:7:in `rails_admin_form_for'