'block-templates' theme support breaks the site editor on classic themes

[abaicus]

Description

Since 6.1.1 core version, adding the block-templates theme support to a classic theme causes the site editor page to load as empty.

Step-by-step reproduction instructions

1. Activate any classic theme (I tried on twentytwenty and a bunch of other themes);
2. Add the following code in the functions.php file:

add_action( 'after_setup_theme', function() {
    add_theme_support( 'block-templates' );
} );

3. Add a templates folder inside the root folder of the theme;
4. Add a index.html file inside of the templates folder with any content - I used the following:

<!-- wp:heading {"textAlign":"center","level":1,"fontSize":"xxl"} -->
<h1 class="has-text-align-center has-xxl-font-size">Our awesome blog</h1>
<!-- /wp:heading -->

5. Go to wp-admin/site-editor.php on the website;
6. The site editor will load a blank page, and there are no errors in the console;

Screenshots, screen recording, code snippet

No response

Environment info

• WordPress core 6.1.1 - it used to work with 6.1.0;
• The same thing happens with Gutenberg installed;
• Happens both in Chrome and Firefox;
• I am on macOS 13.0.1 Ventura;

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[Mamaduka]

I believe this is the same issue as described in #45862.

[abaicus]

Indeed, @Mamaduka! Thanks for pointing it out. I will close this one as it seems a PR has already been made there 👍🏻

Thank you!