NoneType object has no attribute startswith

```
INFO: Writing failed: %s
Executing:   0%|          | 0/56 [00:00<?, ?cell/s]INFO: Executing notebook with kernel: python3
Executing: 100%|██████████| 56/56 [30:31<00:00, 32.70s/cell]
ERROR: 'NoneType' object has no attribute 'startswith'
```

Not sure where this comes from but I can see the error message after `sos-papermill --execute` from time to time.