Return the ColorWindow by ref, which also
allows to drop a null check in
ColorListBox::ToggleHdl.
Change-Id: Ife262e607eb090bee39fda1a9f497bdf1baf263d
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/207333
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>