module Roda::RodaPlugins::NotFound::InstanceMethods

def call

the not_found handler.
If routing returns a 404 response with an empty body, call 
def call
  result = super
  if result[0] == 404 && (v = result[2]).is_a?(Array) && v.empty?
    @_response.headers.clear
    super{not_found}
  else
    result
  end
end

def not_found

break things.
the plugin without defining a not_found handler doesn't
Use an empty not_found_handler by default, so that loading 
def not_found
end