Error on startup in mount_sidebar

Here's the error:

```
Task exception was never retrieved
future: exception=KeyError('groups',)>
Traceback (most recent call last):
File "app.py", line 100, in component_did_mount
yield from self.mount_sidebar(executor)
File "app.py", line 109, in mount_sidebar
loop.run_in_executor(executor, self.store.load_users)
File "/Users/roveo/.pyenv/versions/3.6.6/lib/python3.6/concurrent/futures/thread.py", line 56, in run
result = self.fn(*self.args, **self.kwargs)
File "/Users/roveo/sclack/sclack/store.py", line 99, in load_groups
self.state.groups = self.slack.api_call('mpim.list')['groups']
KeyError: 'groups'
```