Are you submitting a bug report or a feature request?

bug report

What is the current behavior?

When using FormSpy, the onChange is called during rendering, and throws an error if you use setState.

<FormSpy
  subscription={{ values: true }}
  onChange={change => {
    // fired during rendering, calling a `useState` setter fails
    setValues(change.values);
  }}
/>

Warning: Cannot update a component (`App`) while rendering a different component (`FormSpy`). To locate the bad setState() call inside `FormSpy`, follow the stack trace as described in https://fb.me/setstate-in-render
    in FormSpy (at App.js:23)
    in form (at App.js:13)
    in ReactFinalForm (at App.js:10)
    in App (at src/index.js:9)
    in StrictMode (at src/index.js:8)

What is the expected behavior?

No error.

Sandbox Link

https://codesandbox.io/s/react-final-form-formspy-hltnv?file=/src/App.js

What's your environment?

    "final-form": "4.20.0",
    "react": "16.13.1",
    "react-dom": "16.13.1",
    "react-final-form": "6.5.0",

Other information

The issue seems to be that onChange in the useFormState is called in the useState init function, which means during rendering:

This is part of a range of error messages introduced in react 16.13, and related to an existing issue (#751)

A workaround would be to put any setState on the onChange in a setTimeout(... ,0), but this feels like a hack, and not clear to new users.