DataViewWidget should not allow data_model to be None or handle that case

At least at the point where `_create()` is called, the `data_model` should be non-`None`, otherwise we get errors with basic operations like sizing the view. Should possibly consider just having `allow_none=False` for the `data_model` trait.

Alternatively, we could make the underlying Qt model robust to missing data models if we can provide sensible defaults.