Uploadcare file / image widget not working when embedded in list widget

[rordi]

Describe the bug
Using the file or image widget within the list widget does not work when Netlify CMS is configured to use Uploadcare as the media library. The button "Choose an image" or "Choose a file" produces the following JS error:

react-dom.production.min.js:17 Uncaught TypeError: Cannot read property 'toJS' of null
    at Hu (index.js:48)
    at Object.show (index.js:117)
    at R (mediaLibrary.js:67)
    at index.js:9
    at bindActionCreators.js:3
    at t (withFileControl.js:129)
    at Object.<anonymous> (react-dom.production.min.js:15)
    at Object.invokeGuardedCallback (react-dom.production.min.js:16)
    at Object.invokeGuardedCallbackAndCatchFirstError (react-dom.production.min.js:16)
    at C (react-dom.production.min.js:20) 

To Reproduce

1. adjust config.yml to use Uploadcare as the media library

media_library:
  name: uploadcare
  config:
    publicKey: .....

1. add list widget to config.yml with field that uses either the image or file widget

collections:
  - name: "page"
    label: "Page"
    folder: "site/content"
    slug: "{{slug}}"
    create: true 
    fields:
      - { label: "Titel", name: "title", widget: "string", tagname: "h1" }
      - { label: "Date", name: "date", widget: "datetime" }
      - { label: "Content", name: "body", widget: "markdown" }
      - { label: "Media (works)", name: "image_single", widget: "image" }
      - label: "Media in list (does not work)"
        name: "image_list"
        widget: "list"
        field: {label: "Image", widget: "image", default: "", required: false }
        required: false

Expected behavior
The Uploadcare widget should open.

Applicable Versions:

• Netlify CMS version: 2.1.2
• Git provider: GitHub
• OS: macOS Sierra 10.12.4
• Browser version: chrome 69
• Node.JS version: 8.9.1

CMS configuration

backend:
  name: git-gateway
  branch: master
  accept_roles:
    - admin
    - editor

media_library:
  name: uploadcare
  config:
    publicKey: 51fe52694febf733ab27

collections: # A list of collections the CMS should be able to edit

  - name: "page"
    label: "Seiten"
    folder: "site/content"
    slug: "{{slug}}"
    create: true # Allow users to create new documents in this collection
    fields:
      - { label: "Titel", name: "title", widget: "string", tagname: "h1" }
      - { label: "Publikationsdatum", name: "date", widget: "datetime" }
      - { label: "Seiteninhalt", name: "body", widget: "markdown" }

  - name: "project"
    label: "Projekte"
    folder: "site/content/projects"
    slug: "{{slug}}"
    create: true # Allow users to create new documents in this collection
    fields:
      - { label: "Titel", name: "title", widget: "string", tagname: "h1" }
      - { label: "Publikationsdatum", name: "date", widget: "datetime" }
      - { label: "Rangfolge", name: "position", widget: "number", default: 20 }
      - { label: "Nicht auf Website publizieren", name: "draft", widget: "boolean", default: true, required: false }
      - { label: "Jahr", name: "Year", widget: "date", format: "YYYY", required: false }
      - { label: "Referenz-URL", name: "reference", widget: "string", required: false }
      - { label: "Konzept", name: "concept", widget: "boolean", default: false, required: false }
      - { label: "Planung", name: "planning", widget: "boolean", default: false, required: false }
      - { label: "Umsetzung", name: "implementation", widget: "boolean", default: false, required: false }
      - label: "Aufgabenbereiche"
        name: "activities"
        widget: "list"
        field: {label: "Aufgabenbereich", widget: "select", options: ['Shopdesign', 'Bürodesign', 'Rendering', 'Ausstellungsgestaltung', 'Schaufenstergestaltung', 'Eventgestaltung', 'Messegestaltung', 'Signaletik', 'Display', 'Point-of-Sale']}
        required: false
      - { label: "Teaser-Bild", name: "image", widget: "image" }
      - { label: "Teaser-Bild Titel / Alt", name: "imageAlt", widget: "string", required: false }
      - { label: "Teaser-Text", name: "teaser", widget: "text" }
      - label: "Projektbilder"
        name: "projectpictures"
        widget: "list"
        field: {label: "Bild", widget: "image", default: "", required: false }
        required: false
      - { label: "Seiteninhalt", name: "body", widget: "markdown", required: false }

  - name: "team"
    label: "Mitarbeiter"
    folder: "site/content/team"
    slug: "{{slug}}"
    create: true # Allow users to create new documents in this collection
    fields:
      - { label: "Publikationsdatum", name: "date", widget: "datetime" }
      - { label: "Rangfolge", name: "position", widget: "number", default: 20 }
      - { label: "Nicht auf Website publizieren", name: "draft", widget: "boolean", default: true, required: false }
      - { label: "Von Google-Suche ausschliessen", name: "robotsExclude", widget: "boolean", default: false, required: false }
      - { label: "Vor- & Nachname", name: "title", widget: "string", tagname: "h1" }
      - { label: "Funktion", name: "function", widget: "string", tagname: "h3" }
      - { label: "kurzes Zitat", name: "quote", widget: "string", tagname: "h4", required: false }
      - { label: "E-Mail", name: "email", widget: "string", required: false }
      - { label: "Telefon", name: "phone", widget: "string", required: false }
      - { label: "Mitarbeiter-Bild", name: "image", widget: "file" }
      - { label: "Seiteninhalt", name: "body", widget: "markdown", required: false }

  - name: "job"
    label: "Jobs"
    folder: "site/content/jobs"
    slug: "{{slug}}"
    create: true # Allow users to create new documents in this collection
    fields:
      - { label: "Nicht auf Website publizieren", name: "draft", widget: "boolean", default: true, required: false }
      - { label: "Publikationsdatum", name: "date", widget: "datetime" }
      - { label: "Stellenbezeichnung", name: "title", widget: "string", tagname: "h1" }
      - { label: "Seiteninhalt", name: "body", widget: "markdown" }

  - name: "customer"
    label: "Kundenliste"
    folder: "site/content/customers"
    slug: "{{slug}}"
    create: true # Allow users to create new documents in this collection
    fields:
    - { label: "Kunde", name: "title", widget: "string", tagname: "h1" }
    - { label: "Nicht auf Website publizieren", name: "draft", widget: "boolean", default: true, required: false }

[nd0ut]

Thank you!

I was going to go deeper into the External Media API so I'll try to fix this bug. I guess it's the same as #1602 (comment)