TypeError: e.join is not a function

To Reproduce

Steps to reproduce the behavior:

1. Login
2. Open any post

Screenshots

Applicable Versions:

• Netlify CMS version: 2 (https://cdn.jsdelivr.net/npm/netlify-cms@2/dist/netlify-cms.min.js)
• Git provider: Gitlab
• Generator: Hexo

CMS configuration

backend:
  name: gitlab
  repo: xxxx/xxxxx # Path to your GitLab repository

display_url: https://www.xxxx.com

media_folder: "images/uploads"
public_folder: "/images/uploads"

collections:
  - name: "posts" # Used in routes, e.g., /admin/collections/blog
    label: "Post" # Used in the UI
    folder: "source/_posts" # The path to the folder where the documents are stored
    create: true # Allow users to create new documents in this collection
    slug: "{{slug}}" # Filename template, e.g., YYYY-MM-DD-title.md
    fields: # The fields for each document, usually in front matter
      - {label: "Layout", name: "layout", widget: "hidden", default: "blog"}
      - {label: "Title", name: "title", widget: "string"}
      - {label: "Categories", name: "categories", widget: "list"}
      - {label: "Tags", name: "tags", widget: "list"}
      - {label: "Publish Date", name: "date", widget: "datetime"}
      - {label: "Body", name: "body", widget: "markdown"}

[tomrutgers]

Your list widgets (tags and categories) do not have field or fields defined.

- label: "Tags"
  name: "tags"
  widget: "list"
  field: {label: "Tag", name: "tag_name", widget: "string"}

[steakscience]

I’m also running into this issue with a list widget in my custom widget:

fields: [{label: "Images", name: "id", widget: "list", field: {label: "Image", name: "galleryImage", widget: "image"}}],

Does anyone know a fix for this?