PHP Notice - Undefined Index

[Sonhudo]

Hello!

I'm using 1.6.1.18 with PHP 5.6.40,  MySQL 5.6.44 

I'm hunting for errors on my site and in my log file I have tones of instances of this same error:

[16-Jan-2019 22:23:13 Greenwich] PHP Notice:  Undefined index: hook in /home/rieraalta/public_html/cache/smarty/compile/cb/6f/4f/cb6f4f62fc1fecfa8a51eb7b687bf99ab9421a45.file.hook.tpl.php on line 34

So I went looking for file and found this to be line 34:

<?php ob_start();?><?php echo mb_convert_encoding(htmlspecialchars($_smarty_tpl->tpl_vars['hook']->value, ENT_QUOTES, 'UTF-8', true), "HTML-ENTITIES", 'UTF-8');?>

 

I'm not an expert on the matter and I really can't make anything of this, I was wondering If someone out there can point me to the right direction. 

Thanks