TypeError: Cannot read property 'map' of undefined

[machadoluiz]

Describe the bug
As you can see below, the code works for the most part, but specifically in the Preview pane, when I add a group element (list) in the CMS, it breaks for some reason. I've noticed that the issue happens in the custom preview template, in this line, but I wasn't able to see anything wrong in there. Is there a possible fix to it, please?

I'm posting it here because I couldn't find anyone that had a similar issue. I'm sorry if that's not the right place for this kind of issue, if needed, I can delete.

To Reproduce
I created this public repo with just the code needed, so anyone can easily reproduce.
You can also see this site here.

Expected behavior
Be able to add this group element without breaking the Preview pane.

Screenshots

When adding a group element

When adding a subgroup element (nested list) - it works as expected

Applicable Versions:

• Netlify CMS version: netlify-cms-app@2.12.17
• Git provider: git-gateway
• Browser version: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.89 Safari/537.36

CMS configuration

publish_mode: simple
slug:
  encoding: unicode
  clean_accents: false
  sanitize_replacement: "-"
backend:
  name: git-gateway
  branch: master
media_folder: static/images
public_folder: /images
collections:
  - name: pages
    label: Pages
    files:
      - file: src/pages/index.md
        label: Index
        name: index
        fields:
          - label: Groups
            name: groups
            widget: list
            fields:
              - label: Title
                name: title
                widget: string
              - label: Subgroups
                name: subgroups
                widget: list
                fields:
                  - label: Subtitle
                    name: subtitle
                    widget: string
    publish: true
    sortableFields:
      - commit_date
      - commit_author
    view_filters: []

Additional context