Comment # 16 on bug 802038 from Emotions Binary

(In reply to Guido Berhoerster from comment #12)
> OK, closing for now. Please reopen if you find something out allowing me to
> reproduce or run into this on 12.3 or Factory.

The issue always happens if you set "greeter-hide-users=true" in the
lightdm.conf file.

As you can see from the log files, 

[+8.37s] DEBUG: Session pid=5697: Started with service 'lightdm', username
'(null)'

And the error then.

Otherwise, if "greeter-hide-users=false", "(null)" will be replaced with the
last logged-in user and everything works @ first try.