Crashing whilst sending a notification

```
ntfy send "steve"
Traceback (most recent call last):
File "/usr/bin/ntfy", line 8, in
sys.exit(main())
File "/usr/lib/python3.8/site-packages/ntfy/cli.py", line 362, in main
return notify(
File "/usr/lib/python3.8/site-packages/ntfy/__init__.py", line 29, in notify
for backend in config.get('backends', ['default']):
TypeError: 'NoneType' object is not iterable
```
Config file:
```
---
backends:
pushbullet:
access_token:
simplepush:
key:
```
I had this all working last night, so it is probably something I have done.

Simplepush is working, pushbullet was working but I think their server is down, any ideas?

:Edit: pushbullet now working, it was a glitch at their end.

:edit2:
```
ntfy -b default send 'hi world'
```
Setting the backend to default works.