Are you submitting a bug report or a feature request?

Bug report

What is the current behavior?

When registering a field for both the array as a whole and individual array items, with array validation and item validation, final-form can throw an error.
Pretty niche use case to want fields for both the array as a whole and individual array items.

https://codesandbox.io/s/react-final-form-field-arrays-forked-vjf4lg?file=/index.js
Click one entry in the multi select - Cannot set a numeric property on an object

The error comes from using final-form-arrays, but is thrown from final-form.

What is the expected behavior?

Setting no exception (setIn is setting undefined`) should not blow up here

Sandbox Link

What's your environment?

Final-form 4.12.0
final-form-arrays 1.1.2
react-final-form 4.1.0
react-final-form-arrays 2.0.3

Other information

This is from not setting the ARRAY_ERROR quite as final-form wants it, but requiring that is hard to interop with e.g. yup. Mainly reporting this issue to record the error case, rather than because I expect a fix.

Another workaround is not to register fields for array items where not required - when using FieldArray like this, can use field.values to get the value, which is sufficient if we're just trying to read the value.