Method form.focus doesn't work

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

Looks like a bug.

### What is the current behavior?

I take the object `form` from `FormRenderProps` and try to focus on a field by method `form.focus('field_name')`, but it doesn't work.

### What is the expected behavior?

It is possible to focus on the field by method `form.focus()` and field name.

### Sandbox Link

https://codesandbox.io/s/strange-keller-x6c08?file=/src/App.js

### What's your environment?

- `final-form@4.20.1`
- `react-final-form@6.5.2`
- `react@17.0.0`

### Other information