Notice: Undefined index: drupalchat_last_timestamp in drupalchat_poll()

Hello, some time on my site I have a blank page with this code:
{"messages":[],"status":0,"total_messages":0,"last_timestamp":1471859723,"buddylist":{"c-0":{"name":"Public Chatroom","status":"1","p":"http:\/\/www.xxxxxx.it\/sites\/all\/modules\/drupalchat\/themes\/light\/images\/default_room.png"},"total":0}}

In the error log I see:

POSIZIONE	http://www.xxxxxx.it/drupalchat/poll
REFERRER	http://www.xxxxxx.it/user/4069/edit?pass-reset-token=GSS3P2uico2KWVK9ibbLYF8-J9SiEwwN5xcqiFTII5g&destination=drupalchat/poll
MESSAGGIO	Notice: Undefined index: drupalchat_last_timestamp in drupalchat_poll() (linea 717 di /web/htdocs/www.xxxxxx.it/home/sites/all/modules/drupalchat/drupalchat.module).

In this case I was testing the password reset token.