Comment # 2 on bug 72732 from Dave Gilbert

and now understand the perm problem; I was running 

X :2 


  from vt 2, it was starting X visible on vt 3, when I changed to vt3 it
removed the acl on /dev/dri/card0 so that it lost the perms when clients ran
(although the server still had it open).

So still say it's missing that NULL check.