Gutenberg sidebar example does not update meta field in the database

Question

Gutenberg example for creating sidebar https://wordpress.org/gutenberg/handbook/designers-developers/developers/tutorials/plugin-sidebar-0/ does not keep what is entered in the box. After refreshing the page, entered content is gone end text box is empty. I guess it is some kind of REST problem?

I just checked, when updating post, meta field is sent via POST request, therefore it is not a REST problem:

{"meta":{"sidebar_plugin_meta_block_field":"Jozica1"},"content":" ... etc

PHP Code:

add_action( 'init', function () {
    if (is_admin()) {
        wp_register_script( 'gmk-sidebar-js', get_template_directory_uri() . '/js/sidebar-panel.js',
        [ 'wp-plugins', 'wp-edit-post', 'wp-element', 'wp-components', 'wp-data' ]
        );
        wp_register_style( 'gmk-sidebar-style', get_template_directory_uri() . '/css/admin-guten-sidebar.css' );

        register_post_meta( '', 'sidebar_plugin_meta_block_field', array(
            'show_in_rest' => true,
            'single' => true,
            'type' => 'string',
        ) );
    }
} );

Answer 1

Registration of meta field must happen outside of admin area check. I had it enclosed in if (is_admin) check, so it was only happening when it passed this test. This was incorrect.

The question is duplicate and original question and answer are here: Plugin Sidebar is not Saving Meta Attribute to Post/Page after "Update"