(4.18.6) isEqual regression

[sqal]

Are you submitting a bug report or a feature request?

Bug report

What is the current behavior?

Second parameter passed to custom isEqual function is sometimes undefined

Please check reproduction example:

1. Go to https://codesandbox.io/s/final-form-vanilla-js-demo-irobj
2. Check console logs

You can see that array2 value is sometimes undefined.

What is the expected behavior?

array2 in isEqual function should always be an array.

Other information

Affected version: 4.18.6. It worked correctly in previous versions of Final Form