Exception in execute callback when preempting policy execution

[cdondrup]

When policy execution is preempted the following error sometimes pops up:

[INFO] [WallTime: 1449064461.123582] Preempt on state machine requested!
[INFO] [WallTime: 1449064461.138913] Closest node: Intermediate
[INFO] [WallTime: 1449064461.139138] Using policy execution from Intermediate to Start
[INFO] [WallTime: 1449064461.164741] State machine terminating 'NAVIGATION':'preempted':'preempted'
[INFO] [WallTime: 1449064461.165396] Concurrent state 'NAV_SM' returned outcome 'preempted' on termination.
[ INFO] [1449064461.172598274]: Got new plan
[ WARN] [1449064461.176068739]: DWA planner failed to produce path.
[INFO] [WallTime: 1449064461.187639] Concurrence serviced preempt.
[INFO] [WallTime: 1449064461.188056] Concurrent Outcomes: {'NAV_SM': 'preempted'}
[INFO] [WallTime: 1449064461.188402] State machine terminating 'MONITORED_NAV':'preempted':'preempted'
[INFO] [WallTime: 1449064461.188711] PREEMPTED
[ERROR] [WallTime: 1449064461.194211] Exception in your execute callback: 'NoneType' object has no attribute 'close'
Traceback (most recent call last):
  File "/opt/ros/indigo/lib/python2.7/dist-packages/actionlib/simple_action_server.py", line 299, in executeLoop
    self.execute_callback(goal)
  File "/home/cdondrup/ros_ws/src/strands_navigation/topological_navigation/scripts/execute_policy_server.py", line 253, in executeCallback
    result = self.followRoute(goal.route)
  File "/home/cdondrup/ros_ws/src/strands_navigation/topological_navigation/scripts/execute_policy_server.py", line 298, in followRoute
    result=self.execute_policy(route)
  File "/home/cdondrup/ros_ws/src/strands_navigation/topological_navigation/scripts/execute_policy_server.py", line 337, in execute_policy
    success=self.navigate_to(self.current_action, target)
  File "/home/cdondrup/ros_ws/src/strands_navigation/topological_navigation/scripts/execute_policy_server.py", line 547, in navigate_to
    self.do_reconf_movebase(params, action)
  File "/home/cdondrup/ros_ws/src/strands_navigation/topological_navigation/scripts/execute_policy_server.py", line 657, in do_reconf_movebase
    self.rcnfclient[action].update_configuration(params)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/dynamic_reconfigure/client.py", line 219, in update_configuration
    msg    = self._set_service(config).config
  File "/opt/ros/indigo/lib/python2.7/dist-packages/rospy/impl/tcpros_service.py", line 435, in __call__
    return self.call(*args, **kwds)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/rospy/impl/tcpros_service.py", line 528, in call
    transport.close()
  File "/opt/ros/indigo/lib/python2.7/dist-packages/rospy/impl/tcpros_base.py", line 807, in close
    self.socket.close()
AttributeError: 'NoneType' object has no attribute 'close'

This seems to be nothing major as it does not break the system but shows that there is at least one uncaught exception somewhere.