Spree::Admin::ShipmentsController.class_eval do
  def comments
    shipment
    @comment_types = Spree::CommentType.where(:applies_to => "Shipment")
  end
end

Spree::Api::ShipmentsController.class_eval do
  def comments
    shipment
    @comment_types = Spree::CommentType.where(:applies_to => "Shipment")
  end
end

Fix namespace for Shipments Controller
Fix namespace for Shipments Controller
