Comment 1 for bug 1572507

it turned out that lswitch table is empty

I got another exception related to this problem:

2016-04-20 14:07:34.277 DEBUG RyuDFAdapter [-] move onto config mode from (pid=1194) hello_handler /usr/local/lib/python2.7/dist-packages/ryu/controller/ofp_handler.py:185
2016-04-20 14:07:34.278 DEBUG RyuDFAdapter [-] switch features ev version=0x4,msg_type=0x6,msg_len=0x20,xid=0x7b319b5a,OFPSwitchFeatures(auxiliary_id=0,capabilities=79,datapath_id=267145071754569,n_buffers=256,n_tables=254) from (pid=1194) switch_features_handler /usr/local/lib/python2.7/dist-packages/ryu/controller/ofp_handler.py:196
2016-04-20 14:07:34.285 ERROR ryu.controller.controller [-] Error in the datapath 0000f2f78f1afd49 from ('10.100.100.8', 60524)
2016-04-20 14:07:34.285 ERROR ryu.lib.hub [-] hub: uncaught exception: Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/ryu/lib/hub.py", line 52, in _launch
    func(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/ryu/controller/controller.py", line 402, in datapath_connection_factory
    datapath.serve()
  File "/usr/local/lib/python2.7/dist-packages/ryu/controller/controller.py", line 320, in serve
    self._recv_loop()
  File "/usr/local/lib/python2.7/dist-packages/ryu/controller/controller.py", line 115, in deactivate
    method(self)
  File "/usr/local/lib/python2.7/dist-packages/ryu/controller/controller.py", line 225, in _recv_loop
    handler(ev)
  File "/opt/stack/dragonflow/dragonflow/controller/ryu_base_app.py", line 151, in switch_features_handler
    self.dispatcher.dispatch('switch_features_handler', ev)
  File "/opt/stack/dragonflow/dragonflow/controller/dispatcher.py", line 43, in dispatch
    handler(*args, **kwargs)
  File "/opt/stack/dragonflow/dragonflow/controller/dhcp_app.py", line 75, in switch_features_handler
    self._install_flows_on_switch_up()
  File "/opt/stack/dragonflow/dragonflow/controller/dhcp_app.py", line 427, in _install_flows_on_switch_up
    self.add_local_port(port)
  File "/opt/stack/dragonflow/dragonflow/controller/dhcp_app.py", line 343, in add_local_port
    if not self._is_dhcp_enabled_on_network(lport, network_id):
  File "/opt/stack/dragonflow/dragonflow/controller/dhcp_app.py", line 288, in _is_dhcp_enabled_on_network
    subnet = self._get_subnet_by_port(lport)
  File "/opt/stack/dragonflow/dragonflow/controller/dhcp_app.py", line 271, in _get_subnet_by_port
    subnets = l_switch.get_subnets()
AttributeError: 'NoneType' object has no attribute 'get_subnets'