404 Error on a preloaded route when creating a new template part.

[Hug0-Drelon]

Description

When adding a new template part in the admin screen, with the request https://myblog.local/wp-admin/post-new.php?post_type=wp_template_part, WordPress preloads data here. Then there is a call to the route /wp/v2/template-parts/\d+ with a digit as template part identifier, which is filtered by Gutenberg here.
Then the gutenberg_get_block_template() function is called and returnnull because it expects a string identifier (e.g. twentytwentytwo//header) here. This leads to a 404 error in the previous get_item() function.
Although the template part is already created as auto-draft (thanks to WordPress and the call to get_default_post_to_edit() here).

This shouldn't return an error if we expect that wp_template_part post type works like any others.
Also, it prevents anyone to add query parameters to the https://myblog.local/wp-admin/post-new.php?post_type=wp_template_part request to get any information or to add any default value when a template part is created for instance.

Step-by-step reproduction instructions

1. Go to Appearance >> Template Parts.
2. Click on "Add New".
3. Use a debugger to see what happen here .

Indeed, the problem can't be seen in the interface.

Screenshots, screen recording, code snippet

No response

Environment info

WordPress 5.8.2, Gutenberg 11.8.3, TwentyTwentyTwo theme.

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

[jordesign]

As Template Parts are both handled differently, and have a different post type slug now - I'll close this issue as no longer reproducible.