Nested field arrays break data.slice

Describe the bug data.slice is not a function when removing multiple sub-elements in nested field arrays.

To Reproduce Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/react-hook-form-defaultvalues-4o5iu
2. Click on “Add element”
3. Click on “add subelement” twice
4. Click on “remove subelement” twice
5. After the second “remove subelement” the app breaks with data.slice is not a function

Codesandbox link (Required) https://codesandbox.io/s/react-hook-form-defaultvalues-4o5iu

Expected behavior Sub-elements should be removed without an error.

Additional context If I add another element before removing the second sub-element it works fine. It seems like after removing one nested field array the state is left in a bad state.

[bluebill1049]

https://react-hook-form.com/api#useFieldArray

it’s due to register not to get invoked inside map when using with FormContext.

[rnnyrk]

Thanks for this one, also missed it in the docs but this resolved the issue.