Update admin.py
Update admin.py

Fix bug: changing sort order of top level tree items (tree items without a parent).
caktus/django-sitetree,PukeCloud/django-sitetree,idlesign/django-sitetree,PukeCloud/django-sitetree,idlesign/django-sitetree,ikresoft/django-sitetree,caktus/django-sitetree,RamezIssac/django-sitetree,ikresoft/django-sitetree,RamezIssac/django-sitetree