AttributeError:  'NoneType' object has no attribute 'domain'

https://sentry.io/anze-fp/orange/issues/195201536/

```
AttributeError:  'NoneType' object has no attribute 'domain'
  File "C:\Python34\lib\site-packages\Orange\widgets\data\owfile.py", line 380, in apply_domain_edit

    chain([(at, 0) for at in self.data.domain.attributes],

 'NoneType' object has no attribute 'domain'
```