Flint: Exception log during FLint termination with ctrl-c

Probably a piece of code which have to be updated cause of the changes of the BLISS RPC structure.

Traceback (most recent call last):
  File "src/gevent/greenlet.py", line 906, in gevent._gevent_cgreenlet.Greenlet.run
  File "/users/valls/workspace/bliss.git/bliss/flint/helper/rpc_server.py", line 60, in _task
    gevent.wait([stop, task], count=1)
  File "/nobackup/lvalls1/valls/Software/miniconda3/envs/blissenv/lib/python3.7/contextlib.py", line 119, in __exit__
    next(self.gen)
  File "/users/valls/workspace/bliss.git/bliss/flint/helper/rpc_server.py", line 40, in safe_rpc_server
    server._rpc_connection.close()
  File "/users/valls/workspace/bliss.git/bliss/comm/rpc.py", line 248, in __getattr__
    return getattr(self._object, name)
AttributeError: 'FlintApi' object has no attribute '_rpc_connection'