If you have just updated to React 17, you might get the following error with Gatsby:
<pre><code>Error: React-Hot-Loader: AppContainer should be patched
</code></pre>
The simple solution to fix this error is to run <code>npm update</code>.
If <code>npm update</code> doesn't change anything, delete your lockfile (<code>yarn.lock</code> or <code>package-lock.json</code>) 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). <code>hot-loader/react-dom(17.0.0-rc.2)</code> and <code>react-hot-loader(4.13.0)</code> have been updated (Which were the original causes of this problem), so there should no longer be any issues going forward.

Gatsby and React 17 - AppContainer should be patched