Very strange, i have not seen this before, a user on Stackoverflow registers the same problem:
https://stackoverflow.com/questions/40849997/wordpress-when-trying-to-save-a-menu-item-in-wp-admin-nav-menus-php-a-download
For testing i would make sure of the following 🌝
Wordpress, Lay Theme & Laytheme's Addons are all up to date ?
Any custom CSS or JS that may interfere is temporarily removed.
Any third-party plugins are de-activated to see if this resolves the issue.
If using a WordPress Cache plugin, disable it or clear your cache.
License key not activated - (in website blank situations or button not available)
https://laytheme.com/troubleshooting.html
Talk soon RichardIf your WordPress Dashboard shows a warning "PHP Update required”
 Please update your PHP version in your webhost control panel.