[LightDM] Dual monitor caused lightdm blank screen when execute reboot/poeroff command.

[Hacksign]

I'm working with an extend monitor connected to my laptop.
when execute reboot or poweroff command.
2 monitor will show an black screen(I can see a cursor on my laptop's 
screen on top-left corner), but reboot/poweroff will not take place.
Here is an error I found in /var/log/lightdm/x-0.log :

[root at XSign hacksign]# cat /var/log/lightdm/x-0.log
X.Org X Server 1.18.3
Release Date: 2016-04-04
X Protocol Version 11, Revision 0
Build Operating System: Linux 4.2.5-1-ARCH x86_64
Current Operating System: Linux XSign 4.5.4-1-ARCH #1 SMP PREEMPT Wed 
May 11 22:21:28 CEST 2016 x86_64
Kernel command line: initrd=\initramfs-linux.img root=/dev/sda3 rw
Build Date: 05 April 2016  05:24:02PM

Current version of pixman: 0.34.0
     Before reporting problems, check http://wiki.x.org
     to make sure that you have the latest version.
Markers: (--) probed, (**) from config file, (==) default setting,
     (++) from command line, (!!) notice, (II) informational,
     (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(==) Log file: "/var/log/Xorg.0.log", Time: Mon May 30 13:08:38 2016
(==) Using config directory: "/etc/X11/xorg.conf.d"
(==) Using system config directory "/usr/share/X11/xorg.conf.d"
xf86: remove device 0 /sys/devices/pci0000:00/0000:00:02.0/drm/card0
failed to find screen to remove

Is anyone knows how to solve this problem ?