react-error-overlay is an overlay which displays when there is a runtime error.
When developing within this package, make sure you run npm start (or yarn start) so that the files are compiled as you work.
This is ran in watch mode by default.
If you would like to build this for production, run npm run build:prod (or yarn build:prod).
If you would like to build this one-off for development, you can run NODE_ENV=development npm run build (or NODE_ENV=development yarn build).