With nullptr a parent is searched for and assigned (unless NoParent is
set which means no parent)

since...

commit dd46727b
Author: Caolán McNamara <caolanm@redhat.com>
Date:   Tue Apr 5 15:27:38 2016 +0100

    Resolves; tdf#87120 no keyboard navigation inside floating windows

    lets try and treat these the same as we do normal toplevels
    like dialogs if they popup with GrabFocus.

    This way focus can be set on widgets inside the floating windows, and
    so keyboard traversal of widgets etc all works.

I believe an active menu is allowed as a parent (which I'll investigate and see
if we can fix or revert that if necessary), but its good practice to explicitly
set the right parent rather than depending on what happens to be on top anyway.

Change-Id: I744f6d9bc133058a4a9db94d6c27d2e36e22179e