Cannot delete submission with empty File field.

Steps:

1. Go to Content/Content structure.
2. Click Create button and select Form.
3. Create form with File and Button fields.
4. Visit form (frontend).
5. Do not choose file in File field and submit.
6. Go to the form submissions in admin interface.
7. Delete submission.

Actual result:
Cannot delete submission, error occurs, see attached screen.

Expected result:
Submission is deleted.