schema-transformer crash on delete of network

R1.06 branch

 2369 def delete_project_virtual_network(self, idents, meta):
 2370 network_name = idents['virtual-network']
 2371 self.current_network_set |= VirtualNetworkST.delete(network_name)
 2372 self.current_network_set.discard(network_name)
 2373 # end delete_project_virtual_network
self = <schema_transformer.to_bgp.SchemaTransformer object>
self.current_network_set = set([])
global VirtualNetworkST = <class 'schema_transformer.to_bgp.VirtualNetworkST'>
VirtualNetworkST.delete = <bound method __metaclass__.delete of <class 'schema_t
ransformer.to_bgp.VirtualNetworkST'>>
network_name = 'default-domain:monitoring:monitoring-s-orccld-0001-my_network-eubu4djkr2sh'
<type 'exceptions.TypeError'>: unsupported operand type(s) for |=: 'set' and 'NoneType'

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/schema_transformer/to_bgp.py", line 3103, in launch_arc
    transformer.process_poll_result(result)
  File "/usr/lib/python2.7/dist-packages/schema_transformer/to_bgp.py", line 2781, in process_poll_result
    func(idents, meta)
  File "/usr/lib/python2.7/dist-packages/schema_transformer/to_bgp.py", line 2371, in delete_project_virtual_network
    self.current_network_set |= VirtualNetworkST.delete(network_name)
TypeError: unsupported operand type(s) for |=: 'set' and 'NoneType'