[shondsouza]

Problem

Many APIs return server-side validation errors as a list of messages per field. Django REST Framework is one example:

{ "errors": { "title": ["The title is too short", "The title must be unique"] } }

setSubmissionErrors treated those lists as <ArrayInput> items and set errors on title.0 and title.1. The title input turned red but showed no message (#8278).

The same function also crashed with TypeError: Cannot convert undefined or null to object when a field's value was null, and crashed when it was undefined. For example, { title: 'Required', body: null } triggers it.

Solution

In setSubmissionErrors:

• A field that maps to an array of strings now gets a single error on the field itself, with the messages joined by a comma.
• null and undefined values, and empty arrays, are ignored.
• Arrays of objects are still treated as <ArrayInput> item errors, as before.

Trade-off: an <ArrayInput> holding scalar items, e.g. tags: ['', 'bad tag'], now gets one combined error on tags instead of one error per item. Mixed arrays (strings and objects) keep the previous behaviour.

How to test

• New unit tests in setSubmissionErrors.spec.ts. Three of them fail without the fix, including the null crash.
• New story Form / ServerSideValidationWithMessageArrays and a matching test in Form.spec.tsx.
• Documented in docs/Validation.md and docs_headless/.../Validation.md.

Closes #8278