17.04 gnome desktop with lightdm: no screen blanking

Question

I use Gnome with Ubuntu 17.04 but I can't use gdm3. With my nvidia Optimus graphics and Prime Sync, the desktop session does not start. It's a bug where gdm3 mistakenly tries to start Wayland. So I am using lightdm. Everything is great, but my screens do not enter power saving mode. I think that gnome requires gdm3 for this to work.

I have installed light-locker. But I can't save settings. If I run light-locker-settings in a shell:

I will report a bug but I can't be alone ....

tim@raffles:~$ light-locker-settings
/usr/share/light-locker-settings/light-locker-settings/light-locker-settings.py:29: PyGIWarning: Gtk was imported without specifying a version first. Use gi.require_version('Gtk', '3.0') before import to ensure that the right version gets loaded.
  from gi.repository import Gtk, GLib, Gio
Traceback (most recent call last):
  File "/usr/share/light-locker-settings/light-locker-settings/light-locker-settings.py", line 176, in apply_cb
    self.apply_settings()
  File "/usr/share/light-locker-settings/light-locker-settings/light-locker-settings.py", line 629, in apply_settings
    self.apply_screen_blank_settings(settings)
  File "/usr/share/light-locker-settings/light-locker-settings/light-locker-settings.py", line 691, in apply_screen_blank_settings
    keyfile = self.get_screensaver_autostart()
  File "/usr/share/light-locker-settings/light-locker-settings/light-locker-settings.py", line 400, in get_screensaver_autostart
    self.get_autostart('screensaver-settings.desktop', defaults)
  File "/usr/share/light-locker-settings/light-locker-settings/light-locker-settings.py", line 358, in get_autostart
    GLib.KeyFileFlags.KEEP_TRANSLATIONS)
GLib.Error: g-key-file-error-quark: Valid key file could not be found in search dirs (2)