Use script in Logstash

Elastic Stack Logstash
6

It's me again, I used this configuration of Logstash and it was working until now, I don't know what changed, but when I use the script in the conf file it doesn't sent logs to elasticsearch, but when I take it away, it works. With the plugin "ruby" in, it says an error like this
Ruby exception occurred: no implicit conversion of nil into String

my conf file:

input {
  udp {
   	port => 5514
   	type => "syslog"
	} 
}
filter {
    grok {
      match => { "message" => "<%{NUMBER:sev}>%{GREEDYDATA:kvlist}" }
    }
    kv {
      source => "kvlist"
      remove_field => ["kvlist"]
    }
    ruby{
      code => 'category = ["0 Kern",
                "1 user",
                "2 mail",
                "3 daemon",
                "4 auth",
                "5 syslog",
                "6 lpr",
                "7 news",
                "8 uucp",
                "9 clock daemon",
                "10 authpriv",
                "11 FTP",
                "12 NTP system",
                "13 log audit",
                "14 log alert",
                "15 cron",
                "16 local0",
                "17 local1",
                "18 local2",
                "19 local3",
                "20 local4",
                "21 local5",
                "22 local6",
                "23 local7"]
gravity = [
                  "0 Emergency",
                  "1 Alert",
                  "2 Critical",
                  "3 Error",
                  "4 Warning",
                  "5 Notice",
                  "6 Informational",
                  "7 Debugging"]
$log = event.get("sev")
$temp = $log.to_i
$i = 0
$y = 0
while ((($i+1)*8)<$temp) do 
  $i+=1 
end 
while (($i * 8) + ($y+1) != $temp) do 
  $y+=1
end
$message = category[$i] + " " + gravity[$y]
event.set("log description", $message)
'
    }
}
output {
      elasticsearch {
        	hosts => [ "localhost:9200" ]
   	     	user => elastic
       		password => elasticlourd
          index => "syslog-%{+YYYY.MM.dd}"
          }
      stdout { codec => rubydebug }
}

I have no idea what could be wrong because it worked until now ..