Cannot change calculator on existing shipping method

[Renatacumming]

Description

When a user tries to change an existing shipping method; wants to change calculator from Weights, to Flat Rate, and gets the 'opps, something went wrong' error. Have recreated this in the Aus Demo Hub with same result.

Expected Behavior

Should be able to change the calculator from one method to another method and save with no error

Actual Behaviour

When the user changes the calculator from one method to another method the 'oops, something went wrong' error appears

Steps to Reproduce

1. Go into an existing shipping method.
2. Try to change the calculator from one method to another method
3. Click Update

Workaround

Create new shipping method and make old shipping method 'display - back office only'

bug-s3: a feature is broken but there is a workaround

[kirstenalarsen]

Is coming up in Bugsnag -https://app.bugsnag.com/yaycode/openfoodnetwork/errors/60a34c07fbf6410007eb6401?event_id=60a34c07007c935b1e800000&i=sk&m=nw

I suspect this could be a regression in this release, perhaps to do with adjustments work @Matt-Yorkley ?

[filipefurtad0]

This looks related and perhaps a duplicate of:
#6300

[Matt-Yorkley]

It doesn't seem to be related to adjustments (it's something to do with units and weights), and also doesn't look like it's new in this release...

[filipefurtad0]

Confirmed duplicate of #6300, closing here.