Cannot return null for non-nullable field Refund.lines

[asonnleitner]

Describe the bug
Unable to remove an order line item from order

To Reproduce
https://drive.google.com/file/d/1n6rn48UbyX6ceIogtJL4xkC8cyU79YUF/view?usp=sharing

Steps to reproduce the behavior:

1. Go to orders, pick an order in PaymentAuthorized state and contains multiple order lines.
2. Click on Modify
3. Reduce quantity on any item, add note and click Preview Changes
4. See error Cannot return null for non-nullable field Refund.lines

Expected behavior
Create a refund without any error

Environment:
https://demo.vendure.io/admin