self.connection.send(iq_obj) AttributeError: 'NoneType' object has no attribute 'send'

[nk]

Gajim disconnected, I asked to connect again and got:

Traceback (most recent call last):
  File "/home/nk/gajim/src/common/xmpp/idlequeue.py", line 132, in process_events
    obj.pollin()
  File "/home/nk/gajim/src/common/xmpp/transports_nb.py", line 351, in pollin
    self._do_receive() 
  File "/home/nk/gajim/src/common/xmpp/transports_nb.py", line 496, in _do_receive
    self.on_receive(received)
  File "/home/nk/gajim/src/common/xmpp/dispatcher_nb.py", line 355, in dispatch
    handler['func'](session,stanza)
  File "/home/nk/gajim/src/common/connection_handlers.py", line 1505, in _LastCB
    self.connection.send(iq_obj)
AttributeError: 'NoneType' object has no attribute 'send'

[asterix]

(In [10297]) prevent a traceback. Fixes #4236