I had a similar problem trying to edit my xorg.conf file.
Here's what worked for me:
At the terminal, type:
You should get a response like
After you should be able to run either
Using X configuration file: "/etc/X11/xorg.conf".
VALIDATION ERROR: Data incomplete in file /etc/X11/xorg.conf.
Undefined Device "(null)" referenced by Screen "Default
Backed up file '/etc/X11/xorg.conf' as '/etc/X11/xorg.conf.backup'
New X configuration file written to '/etc/X11/xorg.conf'
I hope this helps....