Generating notice on wp admin

Hello,
i am using this plugin for my theme, and this plugin is generating error after update to 4.7.8, this error:

<b>Notice</b>: Undefined index: contract_key in <b>/var/www/html/xxxx/wp-content/plugins/newsletter/plugin.php</b> on line <b>1562</b>
I have tested with wordpress default theme too and i have got same errors, and also i have noticed i was using 4.7.6 version before and it was working without any errors.