If you have just updated to React 17, you might get the following error with Gatsby:

Error: React-Hot-Loader: AppContainer should be patched

The simple solution to fix this error is to run npm update.

If npm update doesn't change anything, delete your lockfile (yarn.lock or package-lock.json) and re-run your package manager.

Under the hood, this is related to how React now uses the new JSX transform. It is used in the latest version of Gatsby (Currently v2.24.5). hot-loader/react-dom(17.0.0-rc.2) and react-hot-loader(4.13.0) have been updated (Which were the original causes of this problem), so there should no longer be any issues going forward.