Can't access nested lists or objects in preview templates

**Is your feature request related to a problem? Please describe.**
I can't access the data from a nested list widget using the widgetsFor function in my preview template

**Describe the solution you'd like**
I have a list widget that is nested inside of an object widget which I am unable to access in order to display in my preview template using the widgetsFor function

i.e.
```
# config.yml

...

- name: about
label: About
widget: object
fields:
- name: links
label: Links
widget: list
fields:
- { name: title, label: Link text, widget: string }
- { name: url, label: URL, widget: string }
```