Comment 7 for bug 796578

It seems that the "at" child was already removed in the upstream:
http://mail.gnome.org/archives/commits-list/2011-May/msg03106.html

After removing the child line and compiling the schemas the error goes away.

#glib-compile-schemas /usr/share/glib-2.0/schemas
# gsettings list-recursively org.gnome.desktop.default-applications
org.gnome.desktop.default-applications.terminal exec 'gnome-terminal'
org.gnome.desktop.default-applications.terminal exec-arg '-x'

I have also checked other schemas and none of the others suffers the same issue:
#!/bin/bash
SCHEMAS=`gsettings list-schemas`
for I in $SCHEMAS ; do
 echo "=== $I ==="
 gsettings list-recursively $I

done

Best regards
Michal Ambroz