Describe the bug
When accessing to add a post section I have this message in the Editor :

Error: Minified React error #31; visit https://reactjs.org/docs/error-decoder.html?invariant=31&args[]=Sat%20Mar%2021%202020%2020%3A04%3A50%20GMT%2B0100%20(Central%20European%20Standard%20Time)&args[]= for the full message or use the non-minified dev environment for full errors and additional helpful warnings.

To Reproduce

1. Go to 'collections'
2. Click on 'post'
3. Click on 'new post'
4. See error above

Expected behavior
There should be the preview in here
Screenshots

Applicable Versions:

• netlify-cms-app 2.9.7
• netlify-cms-core 2.12.2
• Git provider: GitHub
• OS: MacOS X
• Chrome