fix for getFieldValue creating a new field in fieldMeta issue

[lulut]

Trying to fix this issue:
#181

[lulut]

Copied from issue log for your convenience:

https://codesandbox.io/s/50x3l7y59n

Steps to reproduce

1. click "Add field" button
2. new row will be created with no issue
3. click "Error Button" button, no impacts to the function since the "list" variable has been initialized as an Array correctly.
4. reload the page
5. this time click "Error Button" first.
6. a new field "list" will be added to fieldMeta
7. click "Add field" button
8. error occurred because list is an 'object' instead of 'Array'

[lulut]

I don't why the check failed at coverage step. Can anyone have a look on this?