Comment 62 for bug 1357090

#60 and #51 solved my issue. Kudos to them

#51:

After trying unsuccessfully to use all the fixes I found online, I finally managed it to fix this problem myself today:

1. Install Gnome Screensaver
    sudo apt-get install gnome-screensaver
2. Replace Light Locker with Gnome Screen Saver
    Do it first for all new users
    sudo vi /etc/xdg/autostart/light-locker.desktop
    Change the line from:
         Exec=light-locker
     to:
         Exec=gnome-screensaver
   and then do the same for every existing user
   su <username>
   vi ~/.config/autostart/light-locker.desktop
3. Hide light-locker-settings from menu for all users
   sudo vi /usr/share/applications/light-locker-settings.desktop
   Add the line:
   Hidden=true

Hope that helps everyone affected by his bug until Light Locker is actually production ready enough to warrant it's place in a LTS release.

#60:

This solution works on my laptop (ubuntu 16.04, but should work on previous OS):
open as root file
/etc/UPower/UPower.conf
and change IgnoreLid=false ==> IgnoreLid=true
save changes, restart system and now it works :-)
You can remove light-locker now.

You can use bash for these:
sudo sed -i -e 's/IgnoreLid=false/IgnoreLid=true/g' /etc/UPower/UPower.conf
sudo apt-get remove --purge -y light-locker light-locker-settings