$ webpack-dev-server --debug --hot --progress --colors 10% building modules 2/2 modules 0 active
Invalid configuration object.
Webpack has been initialised using a configuration object that does not match the API schema.
...
This was not an easy one to understand, but it turned out to be very simple.
When I had CD-ed into the React app folder, I had typed the foldername with only lower case characters. The foldername was originally typed with a capital first letter. Webpack did not like that.
Solution:
cd ..
cd Folder
yarn start
