vpp driver throws an AttributeError exception during interface creation about networking-vpp

[naveenjoy]

This is because the interface creation fails on some occasion and the API is returning an empty list. The driver is accessing the 'retval' attribute of this empty list and raises the error.
bind port - not in list 1bacd36d-6f53-4ea1-8123-0fad16065a89
Creating /tmp/ae9bf805-e971-42c5-9fef-8fae076a2aaf as a port
Created vhost user interface object: []
checking return value for object: []
Error: 'list' object has no attribute 'retval'
Traceback (most recent call last):
File "/usr/lib/python2.7/site-packages/flask/app.py", line 1836, in call
return self.wsgi_app(environ, start_response)
File "/usr/lib/python2.7/site-packages/flask/app.py", line 1820, in wsgi_app
response = self.make_response(self.handle_exception(e))
File "/usr/lib64/python2.7/site-packages/flask_restful/init.py", line 271, in error_router
return original_handler(e)
File "/usr/lib/python2.7/site-packages/flask/app.py", line 1403, in handle_exception
reraise(exc_type, exc_value, tb)
File "/usr/lib64/python2.7/site-packages/flask_restful/init.py", line 268, in error_router
return self.handle_error(e)
File "/usr/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
response = self.full_dispatch_request()
File "/usr/lib/python2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/usr/lib64/python2.7/site-packages/flask_restful/init.py", line 271, in error_router
return original_handler(e)
File "/usr/lib/python2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/usr/lib64/python2.7/site-packages/flask_restful/init.py", line 268, in error_router
return self.handle_error(e)
File "/usr/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
rv = self.dispatch_request()
File "/usr/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request
return self.view_functionsrule.endpoint
File "/usr/lib64/python2.7/site-packages/flask_restful/init.py", line 477, in wrapper
resp = resource(_args, *_kwargs)
File "/usr/lib/python2.7/site-packages/flask/views.py", line 84, in view
return self.dispatch_request(_args, *_kwargs)
File "/usr/lib64/python2.7/site-packages/flask_restful/init.py", line 587, in dispatch_request
resp = meth(_args, *_kwargs)
File "/opt/stack/networking-vpp/networking_vpp/agent/server.py", line 269, in put
args['segmentation_id'])
File "/opt/stack/networking-vpp/networking_vpp/agent/server.py", line 233, in bind_interface_on_host
(iface, props) = self.create_interface_on_host(if_type, uuid, mac)
File "/opt/stack/networking-vpp/networking_vpp/agent/server.py", line 221, in create_interface_on_host
iface = self.vpp.create_vhostuser(path, mac)
File "/opt/stack/networking-vpp/networking_vpp/agent/vpp.py", line 117, in create_vhostuser
return t.sw_if_index
AttributeError: 'list' object has no attribute 'sw_if_index'