Hello,
I just had a look over the provided module.
The problem seems to be the fact that you are not storing the .twig file in the correct location.
Moving the .twig file to /modules/fadoorders/views/PrestaShop/Admin/Common/Grid/Columns/Content/custom_html.html.twig should fix your error (I haven't tested but this is most likely case sensitive too), as highlighted in the official documentation
I have not tried with your specific .twig file, but creating one with the following code: <div>test</div> outputs the result as in my screenshot.
Please let me know if you have any additional queries and I'll try and see if I can help.
