Focus not working with FieldArray and server side validation

### Are you submitting a **bug report** or a **feature request**?

bug

### What is the current behavior?

1. Create a form with a field array (subForm in the demo) and one field (firstName in the demo)
2. Add validation to subForm's fields
2. Add server side validation to firstName
3. Trigger only the server side validation
4. The error is displayed, but firstName isn't focused

### What is the expected behavior?

firstName should be focused

### Sandbox Link

https://codesandbox.io/s/react-final-form-focus-on-first-error-forked-vz1kh?file=/src/validate.js

### What's your environment?

final-form 4.20.2
final-form-arrays 3.0.2
final-form-focus 1.1.2
react 16.13.1
react-dom 16.13.1
react-final-form 6.5.0
react-final-form-arrays 3.1.2