TypeError: Cannot add property noFallthroughCasesInSwitch, object is not extensible

[okozolin]

Get this error 'TypeError: Cannot add property noFallthroughCasesInSwitch, object is not extensible' when run 'npm start'
(of course after running 'npm install'..).
It seems like a recent known issue
facebook/create-react-app#9429

What worked for me (and discussed in the link above) is setting
"noFallthroughCasesInSwitch": true to compilerOptions in my tsconfig.json

You should probably add a note for that in the Readme.