Comment 10 for bug 1554566

CDOQA tests also ran into what appears to be this same issue, while commissioning a Node into 2.0a2.
I was also able to work around it by setting the Nodes network interface, that's used to PXE boot, to match the fabric for that managed subnet (fabric-3 in this case).

2016-03-15 00:18:24 [maasserver] ERROR: ################################ Exception: 'NoneType' object has no attribute 'url' ################################
2016-03-15 00:18:24 [maasserver] ERROR: Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/django/core/handlers/base.py", line 132, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/utils/views.py", line 180, in view_atomic_with_post_commit_savepoint
    return view_atomic(*args, **kwargs)
  File "/usr/lib/python3.5/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File "/usr/lib/python3/dist-packages/maasserver/api/support.py", line 54, in __call__
    response = upcall(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/django/views/decorators/vary.py", line 21, in inner_func
    response = func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/piston3/resource.py", line 190, in __call__
    result = self.error_handler(e, request, meth, em_format)
  File "/usr/lib/python3/dist-packages/piston3/resource.py", line 188, in __call__
    result = meth(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/api/support.py", line 210, in dispatch
    return function(self, request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 768, in get_preseed
    return HttpResponse(get_preseed(node), content_type="text/plain")
  File "/usr/lib/python3/dist-packages/maasserver/preseed.py", line 393, in get_preseed
    release=Config.objects.get_config('commissioning_distro_series'))
  File "/usr/lib/python3/dist-packages/maasserver/preseed.py", line 680, in render_preseed
    node, osystem, release, rack_controller=rack_controller))
  File "/usr/lib/python3/dist-packages/maasserver/preseed.py", line 628, in get_node_preseed_context
    base_url=rack_controller.url)
AttributeError: 'NoneType' object has no attribute 'url'