Cannot read property 'validators' of undefined" in final-form when change version of final-form-arrays

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

Bug report

### What is the current behavior?

Changing version of final-form-arrays in simple example causes error "Cannot read property 'validators' of undefined" in final-form. Steps to reproduce:
1. Open [simple example](https://codesandbox.io/s/kx8qv67nk5) from docs
2. Change version of final-form-arrays
`- "final-form-arrays": "1.1.2",`
`+ "final-form-arrays": "^1.1.2",`
3. Add customer in the list
4. Remove customer from the list with click on ❌

### What is the expected behavior?

Project works properly

### Sandbox Link

[Simple example with changed final-form-arrays version ](https://codesandbox.io/s/react-final-form-field-arrays-forked-2wntc)