Workflow writer cannot write with empty MCO

Created by: martinRenou

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/pyface/ui/qt4/action/action_item.py", line 163, in _qt4_on_triggered
    self.controller.perform(action, action_event)
  File "/usr/local/lib/python2.7/dist-packages/pyface/tasks/action/task_action_controller.py", line 31, in perform
    return action.perform(event)
  File "/usr/local/lib/python2.7/dist-packages/pyface/tasks/action/listening_action.py", line 58, in perform
    method()
  File "/home/martin/force-wfmanager/force_wfmanager/wfmanager_task.py", line 63, in save_workflow
    writer.write(self.workflow, output)
  File "/home/martin/force-bdss/force_bdss/io/workflow_writer.py", line 27, in write
    "id": workflow.mco.bundle.id,
AttributeError: 'NoneType' object has no attribute 'bundle'```