Log out to switch from Intel to NVIDIA GPU with nvidia X server settings will drop to low-graphic mode

CID: 201409-15477 Dell Latitude 3450

With nvidia proprietary driver installed, I can switch from nvidia GPU to Intel GPU with "NVIDIA X Server Settings" tool, just change the setting there and log out.

However, if you try to switch from nvidia GPU to Intel GPU, and log out, it will drop into low-graphic mode.
(It works fine if you reboot it)

Error message can be found in the xorg log:
[ 177.997] Markers: (--) probed, (**) from config file, (==) default setting,
        (++) from command line, (!!) notice, (II) informational,
        (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
[ 177.997] (==) Log file: "/var/log/Xorg.0.log", Time: Fri Oct 30 05:54:04 2015
[ 177.997] (==) Using config file: "/etc/X11/xorg.conf"
[ 177.997] (==) Using system config directory "/usr/share/X11/xorg.conf.d"
[ 177.997] Data incomplete in file /etc/X11/xorg.conf
        Undefined Screen "nvidia" referenced by ServerLayout "layout".
[ 177.997] (EE) Problem parsing the config file
[ 177.997] (EE) Error parsing the config file
[ 177.997] (EE)
Fatal server error:
[ 177.997] (EE) no screens found(EE)
[ 177.997] (EE)
Please consult the The X.Org Foundation support
         at http://wiki.x.org
 for help.
[ 177.997] (EE) Please also check the log file at "/var/log/Xorg.0.log" for additional information.
[ 177.997] (EE)
[ 177.997] (EE) Server terminated with error (1). Closing log file.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: nvidia-346-updates 346.96-0ubuntu0.0.1
ProcVersionSignature: Ubuntu 3.19.0-31.36~14.04.1-generic 3.19.8-ckt7
Uname: Linux 3.19.0-31-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.14.1-0ubuntu3.18
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Oct 30 05:58:36 2015
InstallationDate: Installed on 2015-10-27 (3 days ago)
InstallationMedia: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
SourcePackage: nvidia-graphics-drivers-346-updates
UpgradeStatus: No upgrade log present (probably fresh install)