Zendesk asset helper generating empty string

Question

I am building a custom page in Zendesk with the help of React. I am following this instruction: https://support.zendesk.com/hc/en-us/articles/4421961625370-Develop-Bundle-Embed-Custom-Page-Implementation-with-React

I have this script tag in my .hbs file:

 <script src="{{asset 'bundle.min.js'}}" type="text/javascript" defer></script>

But when I preview the page, it is empty, like a standard React page where there is only root container.

In the sources of the page in preview I can see it like if the asset helper didn't work at all, instead it was just cut out.

 <script src="" type="text/javascript" defer=""></script>

My bundle.min.js is there in the assets folder in the theme, no question about it.

Same problem I have for the css file.

What am I missing or doing wrong?