The bug in TreeLists with black background is NOT from scheduler.
It is from patch 148e489e
Lot of dialogs not set the background, that was the problem.
My fix set the backgound to the default background.

Change-Id: I0f07bf54131aeb177bcf150e6854203c34b6b972
Reviewed-on: https://gerrit.libreoffice.org/15071Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>