php error instead of custom 404 page

Hi,

I set up a custom 404 page using the built-in 404 page in the theme settings. It was working perfectly for a while, but now, it has started displaying a php error instead of the error page itself.

Here is what I get now instead of the error page.

"


Fatal error: Uncaught Error: Call to undefined function
mfn_builder_print() in
/hermes/walnaweb06a/b2072/moo.jmhoffman/advancedir/wp-content/themes/betheme-child/404.php:61
Stack trace:
#0
/hermes/walnaweb06a/b2072/moo.jmhoffman/advancedir/wp-includes/template-loader.php(77):
include()
#1
/hermes/walnaweb06a/b2072/moo.jmhoffman/advancedir/wp-blog-header.php(19):
require_once('/hermes/walnawe...')
#2 /hermes/walnaweb06a/b2072/moo.jmhoffman/advancedir/index.php(17):
require('/hermes/walnawe...')
#3 {main}
thrown in
/hermes/walnaweb06a/b2072/moo.jmhoffman/advancedir/wp-content/themes/betheme-child/404.php
on line 61

"

Can you please help me get my 404 error page working again?

Here is the link to the site:

https://www.advanced-ir.com/

and here is the link that caused the error code to appear (although it appears for any incorrect page url)