Having hidden FieldArrays in FieldArrays can mess up state if outer FieldArray item is deleted

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

Bug report

### What is the current behavior?

https://codesandbox.io/s/react-final-form-nested-arrays-validation-32he5

1. Expand the first company's customer section
2. See that all customers are editable (typing in customer fields is possible, company name is editable)
3. Delete the company that is currently expanded (red x next to company name)
4. The customer fields are uneditable, the fields contain the deleted company's customers

### What is the expected behavior?

All fields remain editable

### Sandbox Link

https://codesandbox.io/s/react-final-form-nested-arrays-validation-32he5

### What's your environment?

Latest chrome