TypeError: Cannot read property 'get' of undefined

**Describe the bug**

I have enabled i18n for content authoring. I believe I should be able to save a post in the primary language without adding all the translations at the same time. When I try to save, I get no visible error. Instead, I get this error in the console.

**To Reproduce**

0. On a site configured as my config linked below
1. Go to `/admin`
2. Click on 'New Blog Post'
3. Complete the form for the English post
4. Try and click on 'Save'
5. See error in console

**Expected behavior**

0. On a site configured as my config linked below
1. Go to `/admin`
2. Click on 'New Blog Post'
3. Complete the form for the English post
4. Try and click on 'Save'
5. Saves only the translation you've created

**Screenshots**



**Applicable Versions:**

- Netlify CMS: 2.10.63
- Git provider: GitHub
- OS: Mac
- Browser version: Version 86.0.4240.80 (Official Build) (x86_64)

**CMS configuration**

https://github.com/Nexmo/deved-platform/blob/i18n-netlify-cms/static/admin/config.yml

**Additional context**

If I complete all the fields for all the languages, it then lets me save!