[2.0rc1] after upgrade from 2.0b3, Error on request (13) subnet.list: 'NoneType' object is not iterable

[Andres Rodriguez]

After an upgrade from 2.0b3, we see this repeatedly:

2016-07-07 00:32:55 [-] 127.0.0.1 - - [07/Jul/2016:00:32:54 +0000] "GET /MAAS/rpc/ HTTP/1.0" 200 284 "-" "provisioningserver.rpc.clusterservice.ClusterClientService"
2016-07-07 00:32:57 [HTTPChannel,79,127.0.0.1] Closing connection: <STATUSES=GOING_AWAY>
2016-07-07 00:32:57 [-] /usr/lib/python3/dist-packages/django/templatetags/future.py:25: django.utils.deprecation.RemovedInDjango19Warning: Loading the `url` tag from the `future` library is deprecated and will be removed in Django 1.9. Use the default `url` tag instead.
2016-07-07 00:32:57 [-] 127.0.0.1 - - [07/Jul/2016:00:32:56 +0000] "GET /MAAS/ HTTP/1.1" 200 1610 "-" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0"
2016-07-07 00:32:57 [-] 127.0.0.1 - - [07/Jul/2016:00:32:57 +0000] "GET /MAAS/combo/ng-tags-input.js?v=2.0.0(rc1+bzr5143) HTTP/1.1" 200 9399 "http://192.168.0.141/MAAS/" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0"
2016-07-07 00:32:57 [-] 127.0.0.1 - - [07/Jul/2016:00:32:57 +0000] "GET /MAAS/combo/yui.js?v=2.0.0(rc1+bzr5143) HTTP/1.1" 200 12635 "http://192.168.0.141/MAAS/" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0"
2016-07-07 00:32:57 [-] 127.0.0.1 - - [07/Jul/2016:00:32:56 +0000] "GET /MAAS/combo/angular.js?v=2.0.0(rc1+bzr5143) HTTP/1.1" 200 58221 "http://192.168.0.141/MAAS/" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0"
2016-07-07 00:32:57 [-] 127.0.0.1 - - [07/Jul/2016:00:32:56 +0000] "GET /MAAS/combo/jquery.js?v=2.0.0(rc1+bzr5143) HTTP/1.1" 200 41724 "http://192.168.0.141/MAAS/" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0"
2016-07-07 00:32:57 [-] 127.0.0.1 - - [07/Jul/2016:00:32:57 +0000] "GET /MAAS/combo/maas-yui.js?v=2.0.0(rc1+bzr5143) HTTP/1.1" 200 10753 "http://192.168.0.141/MAAS/" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0"
2016-07-07 00:32:57 [-] 127.0.0.1 - - [07/Jul/2016:00:32:57 +0000] "GET /MAAS/combo/maas-angular.js?v=2.0.0(rc1+bzr5143) HTTP/1.1" 200 87802 "http://192.168.0.141/MAAS/" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0"
2016-07-07 00:32:57 [HTTPChannel,68,127.0.0.1] Opening connection with IPv4Address(TCP, '127.0.0.1', 39096)
2016-07-07 00:32:58 [-] Error on request (13) subnet.list: 'NoneType' object is not iterable
 Traceback (most recent call last):
   File "/usr/lib/python3.5/threading.py", line 862, in run
     self._target(*self._args, **self._kwargs)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 904, in worker
     return target()
   File "/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
     task()
   File "/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
     task()
 --- <exception caught here> ---
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 246, in inContext
     result = inContext.theWork()
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
     inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
     return self.currentContext().callWithContext(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
     return func(*args,**kw)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 937, in callInContext
     return func(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 602, in call_within_transaction
     return func_outside_txn(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 421, in retrier
     return func(*args, **kwargs)
   File "/usr/lib/python3.5/contextlib.py", line 30, in inner
     return func(*args, **kwds)
   File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 344, in list
     for obj in objs
   File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 344, in <listcomp>
     for obj in objs
   File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 185, in full_dehydrate
     data[field_name] = dehydrate_method(field_obj)
   File "/usr/lib/python3/dist-packages/maasserver/websockets/handlers/subnet.py", line 46, in dehydrate_dns_servers
     return " ".join(sorted(dns_servers))
 builtins.TypeError: 'NoneType' object is not iterable

[Andres Rodriguez (andreserl)]

2016-07-06 22:46:03 [-] Error on request (13) subnet.list: 'NoneType' object is not iterable
 Traceback (most recent call last):
   File "/usr/lib/python3.5/threading.py", line 862, in run
     self._target(*self._args, **self._kwargs)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 904, in worker
     return target()
   File "/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
     task()
   File "/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
     task()
 --- <exception caught here> ---
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 246, in inContext
     result = inContext.theWork()
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
     inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
     return self.currentContext().callWithContext(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
     return func(*args,**kw)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 937, in callInContext
     return func(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 602, in call_within_transaction
     return func_outside_txn(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 421, in retrier
     return func(*args, **kwargs)
   File "/usr/lib/python3.5/contextlib.py", line 30, in inner
     return func(*args, **kwds)
   File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 344, in list
     for obj in objs
   File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 344, in <listcomp>
     for obj in objs
   File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 185, in full_dehydrate
     data[field_name] = dehydrate_method(field_obj)
   File "/usr/lib/python3/dist-packages/maasserver/websockets/handlers/subnet.py", line 46, in dehydrate_dns_servers
     return " ".join(sorted(dns_servers))
 builtins.TypeError: 'NoneType' object is not iterable

2016-07-06 22:46:06 [HTTPChannel,14,127.0.0.1] Closing connection: <STATUSES=GOING_AWAY>
2016-07-06 22:46:07 [-] 127.0.0.1 - - [06/Jul/2016:22:46:06 +0000] "GET /MAAS/zones/ HTTP/1.1" 200 1910 "http://192.168.0.141/MAAS/" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0"
2016-07-06 22:46:13 [-] 127.0.0.1 - - [06/Jul/2016:22:46:12 +0000] "GET /MAAS/ HTTP/1.1" 200 1610 "http://192.168.0.141/MAAS/zones/" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0"
2016-07-06 22:46:14 [HTTPChannel,10,127.0.0.1] Opening connection with IPv4Address(TCP, '127.0.0.1', 34434)
2016-07-06 22:46:20 [HTTPChannel,10,127.0.0.1] Closing connection: <STATUSES=GOING_AWAY>
2016-07-06 22:46:22 [-] 127.0.0.1 - - [06/Jul/2016:22:46:21 +0000] "GET /MAAS/images/ HTTP/1.1" 200 2702 "http://192.168.0.141/MAAS/" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:47.0) Gecko/20100101 Firefox/47.0"
2016-07...