## Versions
- OS: Linux
- GTK+ Version: 3.22.30
- PyGObject Version: 3.26.1
- python-nbxmpp Version: 0.6.4
- Gajim Version: 1.0.1

## Traceback
```
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/nbxmpp/dispatcher_nb.py", line 498, in dispatch
    handler['func'](session, stanza)
  File "/usr/lib/python3/dist-packages/gajim/common/jingle.py", line 111, in _JingleCB
    self._sessions[sid].on_stanza(stanza)
  File "/usr/lib/python3/dist-packages/gajim/common/jingle_session.py", line 353, in on_stanza
    call(stanza=stanza, jingle=jingle, error=error, action=action)
  File "/usr/lib/python3/dist-packages/gajim/common/jingle_session.py", line 447, in __on_session_info
    algo = hash_.getAttr('algo')
AttributeError: 'NoneType' object has no attribute 'getAttr'

```
## Steps to reproduce the problem
...