Deprecated Theme Error Message

Notice: Undefined property: WP_Error::$slug in /home/content/p3nexnas05_data01/68/2098868/html/wp-includes/class-wp-error.php on line 78

I keep receiving this error message with my site. After to speaking to someone, they mentioned that this was because wordpress has updated and my theme has not. Could someone help with this please? This error message has displayed on my home page for a few days now!