Adding user variables can crash variables tab

[LaurensBurger]

Product versie / Product version

2.6.1

Omschrijf het probleem / Describe the bug

Ref: Taiga Utrecht 196

#4069 seems related?
When Objects api (v1) is selected in registration, adding a user variable crashes the variables tab:

ypeError: e.variablesMapping is not iterable
    at onUserDefinedVariableEdit (userDefinedVariableEditHandlers.js:30:54)
    at form-creation-form.js:808:32
    at Proxy.map (<anonymous>)
    at ex (form-creation-form.js:803:73)
    at immer.esm.mjs:1:16170
    at e.produce (immer.esm.mjs:1:16318)
    at immer.esm.mjs:1:16124
    at Object.Oa [as useReducer] (react-dom.production.min.js:170:451)
    at t.useReducer (react.production.min.js:25:274)
    at Di (use-immer.module.js:1:334)

Does not occur when using the new objects api registration or other registration types.

Stappen om te reproduceren / Steps to reproduce

No response

Verwacht gedrag / Expected behavior

No response

Screen resolution

None

Device

None

OS

None

Browser

No response