Values for settings on a custom field type are not displayed after upgrade

[mastrup]

Which Umbraco Forms version are you using? (Please write the exact version, example: 13.4.0)

13.3.3

Which Umbraco version are you using? (Please write the exact version, example: 13.5.2)

13.2.6

Bug summary

When upgrading Umbraco Forms from 10.x.x to 13.x.x, any value stored in a setting on a custom field type is not displayed in the back office.

Specifics

After updating the code that implements a custom field type in Umbraco Forms from v10 to v13, all settings on custom fields disappear.
An example of a field that uses a custom field type in v10.

After upgrading to v13 this value has disappeared.

After the upgrade, I see this on the first boot after the upgrade. After stopping and restarting the project, the error goes away.

Steps to reproduce

I have created a small project where the problem is easy to reproduce - https://github.com/mastrup/FormsUpgradeTest

1. Start the project from the main branch and sign in to the backoffice with [email protected] / 1234567890
2. Create a form in Umbraco and add the custom field type Country DropdownList - Make sure to add a value to the field setting Label.
3. Stop the project and switch to the v13 branch.
4. Start the project once again and locate the form in the backoffice.
5. An error will show. Stop the project and restart it. Locate the form in the backoffice. Observe how the value in the Label field isn't showing.

Expected result / actual result

The saved values of settings in a custom field type must be visible in the back office.