Catchable fatal error

Hello,
I have one subsite that It h says:"Catchable fatal error: Object of class WP_Error could not be converted to string in /home/vietcovn/public_html/wp-includes/formatting.php on line 445" when admin site click menu at link "http://domain.com/wp-admin/nav-menus.php"
Please show me how to fix it.
Thank you