WP 5.0.1 with built-in Gutenberg gives TypeError: "middleware.fields is undefined"

[colorcube]

JS error:

TypeError: "middleware.fields is undefined"

Blocks which uses middleware are not working when the built-in Gutenberg is used with WP 5.0.1.

Maybe because the middleware js is loaded after the editor js. Just a guess.

When installing the gutenberg plugin everything works as expected.

[mohdsayed]

Thanks for trying @colorcube , I just tested with WordPress 5.0.2 however wasn't able to replicate the error. Would you be able to give me steps to produce this error?
You may also take a look at https://github.com/rtCamp/gutenberg-supplements which is created with Gutenberg Fields Middleware for reference.

[colorcube]

I found the problem.
I used wp_cli scaffolding for my custom gutenberg block, like:

wp-cli.phar scaffold block my-custom

In the generated code there's a function to register the block JavaScript with wp_register_script(). So far so good.

But the function will be called with this hook:

add_action('init', 'my_custom_block_init');

gutenberg-fields-middleware uses the hook 'enqueue_block_editor_assets' which seems to be called later.

I changed my hook registration to

add_action('enqueue_block_editor_assets', 'my_custom_block_init');

and everything works fine.