can.exceptions.CanInterfaceNotImplementedError: Unknown interface type "None"

### Describe the bug
```
Traceback (most recent call last):
File "receive_all.py", line 34, in
receive_all()
File "receive_all.py", line 16, in receive_all
with can.Bus() as bus:
File "/opt/conda/lib/python3.8/site-packages/can/interface.py", line 108, in __new__
kwargs = load_config(config=kwargs, context=context)
File "/opt/conda/lib/python3.8/site-packages/can/util.py", line 192, in load_config
bus_config = _create_bus_config(config)
File "/opt/conda/lib/python3.8/site-packages/can/util.py", line 211, in _create_bus_config
raise CanInterfaceNotImplementedError(
can.exceptions.CanInterfaceNotImplementedError: Unknown interface type "None"

```

### To Reproduce
```
pip install python-can
cd python-can/examples
python3 receive_all.py

```

### Expected behavior