V6: useFieldArray with nested useFieldArray renders with fields [empty]

Describe the bug I experience that when I add an entry to a field array with a nested field array and remove it again. Then I get 2 renders of my outer field array. One with [] (length 0) and afterwards one with [empty] (length 1). Then my application crashes since it can’t get the id of undefined.

This is the data I can see with the React Components DevTools:

The issue only exists in v6. Not in v7.

To Reproduce Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/react-hook-form-usefieldarray-nested-arrays-forked-xif3x
2. Click on append
3. Click on delete
4. Click on submit
5. Click on append
6. See that it crashes

Codesandbox link (Required) https://codesandbox.io/s/react-hook-form-usefieldarray-nested-arrays-forked-xif3x

Expected behavior The field array does not contain empty values.

[bluebill1049]

patched again with latest v6.

[bimbeonguyen]

Hello @bluebill1049, this bug is still existing. Step to reproduce: Append -> Append -> Append -> Delete first, second item => the fields return [object, empty] Append -> Append -> Append -> Delete first, second and third item => the fields return [empty, empty, epmty]

https://codesandbox.io/s/react-hook-form-usefieldarray-nested-arrays-forked-xif3x