Shipping fees: Can't change from weight calculator (error 500/snail)

[filipefurtad0]

Description

A user currently sees an error500/snail when changing shipping or enterprise fees calculators, from a weight calculator (kg or lb) to a different one.

Despite the visible snail, changing an enterprise fee from a weight calculator to, say for example, Price Sack, works.

This, however, is not possible if attempting to change a fee calculator for a shipping method, from a weight calculator method to another: attempting to do so results in the visible snail, while keeping the previously weight calculator.

This bug does not affect transaction fees, as the weight calculator is not available for these fees.

Updated - Bugsnag error report
https://app.bugsnag.com/katuma/katuma/errors/5fa1ab275a0a7c0018f04d97?filters[app.release_stage][0][value]=staging&filters[app.release_stage][0][type]=eq&sort=last_seen

Expected Behavior

It should be possible to switch from a weight fee calculator to a different fee calculators.

Actual Behaviour

For enterprise fees: changing from a weight based calculator is possible, but a snail is visible.
For shipping fees: changing from a weight based calculator is not possible - a snail is visible and changes don't take effect.

Steps to Reproduce

1. Set a weight-based fee, on an existing shipping method, choosing kg or lb and attributing a value
2. Press Update
3. Select another Calculator, for example Flat Percent
4. Press Update - Error 500
5. Navigating back will show the previously created weight calculator - changes did not take effect

Repeat the procedure for an Enterprise Fee. Changing fees should be possible, but an error 500 will be visible.

Animated Gif/Screenshot

Workaround

Shipping fee: a workaround would be deleting/deactivating the shipping method and creating another one, but this may raise other issues... for example #5367. So, I'm not sure this is acceptable.
Enterprise fee: after seeing the snail, simply navigate back. Changes should have taken effect.

Severity

bug-s3: a feature is broken but there is a workaround (depending on whether the proposed workaround is acceptable for shipping fees)

Your Environment

• Version used: v3.3.1
• Browser name and version: Firefox 81, Ubuntu 20.
• Operating System and version (desktop or mobile): Desktop Ubuntu 20.04

Possible Fix

Could this have been introduced by #5998? Not sure but it works well in my local system - last updated before v3.2.10. So I think this bug was introduced on this version. Added the regression label.