Fixed non-existing path issue

[rinkalpagdar]

Part of #67445

What?

Site Editor should display a 404 message when you navigate to a non-existing path

Why?

Currently, the Site Editor is showing a black page with no message or content when the user miss-type the link

How?

It will show a 404 message with a button to get back to the site editor.

Testing Instructions

Try navigating to wp-admin/site-editor.php?p=%2Ftemplate-fdgfdgdfg

Screenshots or screencast

fix-non-existing-path.mp4

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: rinkalpagdar <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[talldan]

Interesting idea, though I this can be implemented as part of the routing code to avoid the need to fetch asynchronously. In the site editor the routing is implemented here:

gutenberg/packages/edit-site/src/components/app/index.js

Lines 53 to 60 in 358fe48

	return (
	<RouterProvider
	routes={ routes }
	pathArg="p"
	beforeNavigate={ beforeNavigate }
	>
	<AppLayout />
	</RouterProvider>

The routes come from this file:
https://github.com/WordPress/gutenberg/blob/trunk/packages/edit-site/src/components/site-editor-routes/index.js

The implementation is using the router package as a library. I can see that there's some handling for 404 here:

gutenberg/packages/router/src/router.tsx

Lines 163 to 172 in 8e048a8

	if ( ! result ) {
	return {
	name: '404',
	path: addQueryArgs( path, query ),
	areas: {},
	widths: {},
	query,
	params: {},
	};
	}

I'm not familiar with the code though, so I don't know how to make this display something. Perhaps it should be possible to define a 404 route in the site editor package