None Type error for swda datasets
## Describe the bug
I got `'NoneType' object is not callable` error while calling the swda datasets.

## Steps to reproduce the bug
```python
from datasets import load_dataset
dataset = load_dataset("swda")
```

## Expected results
Run without error

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 2.4.0
- Python version: 3.8.10
