Bug report delete form submission

Description

Steps to procedure:
1. Create new form including file upload element.
2. Create a submission of that form with a uploaded file(s).
3. Modify the value of upload file(s) directly in the database to be empty "" (not null).
Restart IIS or site to reflect the changes (prevent website from caching) because when update a record directly in DB, the updated value would not be consistent with value in cache
4. Delete submission created.

EXPECTED: Deleted successfully.
ACTUAL: Error "Object reference not set to an instance of an object"

For more information, checkout the post on forum:
https://world.episerver.com/forum/developer-forum/episerver-forms/thread-container/2019/4/unable-to-delete-form-submissions-from-code/#203003