[Spree 2.1] Update and Calculate Fees button leads to blank page

[filipefurtad0]

Description

Independently of whether changes were made to an order or not, pressing the Update Fees and Calculate Fees button leads to blank page. Bugsnag is triggered:
https://app.bugsnag.com/yaycode/openfoodnetwork-uk/errors/5eaed1f6da09e400170360a5?event_id=5eaed1f6005b3497ef280000&i=sk&m=nw

Expected Behavior

Changes should be saved to the order, or alerts should be triggered in the case of mandatory fields. Currently, if all items are removed and the Update button is pressed the red banner appears "Line items can't be blank".
If nothing was changed then the button should not do much, I think it just reloads the page.

Actual Behaviour

Pressing the Update Fees and Calculate Fees button leads to a blank page.

Steps to Reproduce

1. As an admin, edit an existing order in page /admin/orders/ordernumber/edit
2. Press the Update and Calculate Fees button

Animated Gif/Screenshot

Workaround

Severity

Your Environment

*Version used:
*Browser name and version: Firefox 75
*Operating System and version (desktop or mobile): Ubuntu 18.04 64

Possible Fix