After a network error, if i try to use a previously opened db connection, an AttributeError exception was raised.

```
File "/home/work/pyenv/revolution/local/lib/python2.7/site-packages/pymongo/collection.py", line 596, in find_one
for result in self.find(spec_or_id, *args, **kwargs).limit(-1):
File "/home/work/pyenv/revolution/local/lib/python2.7/site-packages/pymongo/cursor.py", line 814, in next
if len(self.__data) or self._refresh():
File "/home/work/pyenv/revolution/local/lib/python2.7/site-packages/pymongo/cursor.py", line 763, in _refresh
self.__uuid_subtype))
File "/home/work/pyenv/revolution/local/lib/python2.7/site-packages/pymongo/cursor.py", line 700, in __send_message
**kwargs)
File "/home/work/pyenv/revolution/local/lib/python2.7/site-packages/pymongo/mongo_replica_set_client.py", line 1301, in _send_message_with_response
response = self.__try_read(member, msg, **kwargs)
File "/home/work/pyenv/revolution/local/lib/python2.7/site-packages/pymongo/mongo_replica_set_client.py", line 1214, in __try_read
self.__schedule_refresh()
File "/home/work/pyenv/revolution/local/lib/python2.7/site-packages/pymongo/mongo_replica_set_client.py", line 839, in __schedule_refresh
self.__monitor.schedule_refresh()
AttributeError: 'NoneType' object has no attribute 'schedule_refresh'
```