Programmatic submit does not work in React 17

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

Bug

### What is the current behavior?

Submission using `form.dispatchEvent(new Event('submit', { cancelable: true }))` does not work after upgrading to React 17.

Hit the
 button here to see it do nothing:

https://codesandbox.io/s/react-final-form-external-submit-button-forked-kcrsu?file=/index.js

### What is the expected behavior?

It submits the form, as it did prior to React 17. You can view this behavior by changing the React version in the sandbox.

### Sandbox Link

https://codesandbox.io/s/react-final-form-external-submit-button-forked-kcrsu?file=/index.js

### What's your environment?

RFF 6.5.2
FF 4.20.1
React 17

### Other information

Sandbox is an updated version from https://final-form.org/docs/react-final-form/faq#via-documentgetelementbyid. No other changes.