Line 18 error in Template.php

Kicks out two errors: Undefined variable: post in /wp-monsters/templates.php on line 18

and

Notice: Trying to get property of non-object in /plugins/wp-monsters/templates.php on line 18

I thought adding a line to declare $post would work, but still get the second error.

https://ww.wp.xz.cn/plugins/wp-monsters/