I use your module version 1.0.4 for my site Magento 2.3.3 and I got the error as the screenshot below when cancelling the order on the frontend.
Please let us know how to fix?
This error due to the fact that the config Email address to receive notification in the backend is empty.
Please have a look:
To fix the issue, please edit the file as below:
Or download the file attached and overwrite the current file on your site.