def provider(position)
      content_providers.find_by(position: position) ||
        ContentProviders::Null.new(self, position)
    end
def provider(position)
      content_providers.find_by(position: position) ||
        ContentProviders::Null.new(self, position)
    end
Returns the `ContentProvider` at the given position, or a `Null`
 `ContentProvider` if none exists.