PopUp Pro PHP Warning

Hello,

After submitting the Name and Email in the Newsletter signup form, there is a PHP warning saying:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function 'disable_notification' not found or invalid function name in /home/cornucopiadev/public_html/wp-includes/class-wp-hook.php on line 300

We need help resolving this issue.

Thanks