There is a fatal error on my WordPress site using x theme

I got an email from WP saying there is a fatal error on my site. The error is detected on the page The website host suspects that the problem is related to the website scripts compatibility issues. The Menu on the Appearance tab on WP does not open properly. Is there a solution?

Here are the error details:

Error Details

An error of type E_ERROR was caused in line 155 of the file /home/thedi305/ Error message: Uncaught Error: Call to undefined function fa_all_unicode() in /home/thedi305/
Stack trace:
#0 /home/thedi305/ Menu_Item_Custom_Fields_Map::_fields(‘edit-menu-item-…’, Object(WP_Post), 0, Object(stdClass))
#1 /home/thedi305/ WP_Hook->apply_filters(NULL, Array)
#2 /home/thedi305/ WP_Hook->do_action(Array)
#3 /home/thedi305/ do_action(‘wp_nav_menu_ite…’, ‘2915’, Object(WP_Post), 0, Object(stdClass), 0)
#4 /home/thedi305/ Walker_Nav_Menu_Edit->start_el(’’, Object(WP_Post), 0, Object(stdClass), 0)
#5 /home/thedi305/ CS_Menu_Item_Custom_Fields_Walker->start_el

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