Error on startup: 'NoneType' object is not callable

When I try to start this application, it manages to load the list of channels and chats I have, but then crashed out with the following stacktrace:

```
Traceback (most recent call last):
File "./app.py", line 532, in
app.start()
File "./app.py", line 53, in start
self.urwid_loop.run()
File "/home/gareth/.local/lib/python3.6/site-packages/urwid/main_loop.py", line 286, in run
self._run()
File "/home/gareth/.local/lib/python3.6/site-packages/urwid/main_loop.py", line 384, in _run
self.event_loop.run()
File "/home/gareth/.local/lib/python3.6/site-packages/urwid/main_loop.py", line 1484, in run
reraise(*exc_info)
File "/home/gareth/.local/lib/python3.6/site-packages/urwid/compat.py", line 55, in reraise
value = tp()
TypeError: 'NoneType' object is not callable
```