check if method exist

lib/common/models/vulnerability/output.rb

@@ -10,6 +10,7 @@ class Vulnerability
       references.each do |key, urls|
         methodname = "url_#{key}"
         urls.each do |u|
+          next unless respond_to?(methodname)
           url = send(methodname, u)
           puts "    Reference: #{url}" if url
         end