Are you submitting a bug report or a feature request?

Bug report

What is the current behavior?

If I mount a Field with an async validator, then it throws an error "Cannot read property 'active' of undefined". This issue started in version 4.19.1 and is still present in 4.20.2

What is the expected behavior?

Should not throw an error

Sandbox Link

https://codesandbox.io/s/keen-shadow-hppmc?file=/src/index.js

What's your environment?

OS: Windows 10 64-bit
Node: v14.16.0
final-form: v4.20.1
react-final-form: v6.5.1
react: v16.12.0