I tried to add a cluster as release with Picard and the plugin and get this error on the second page of the add form.
mediums.0 isn’t defined, do your indexes start at 0?
I have no idea what it means and cannot guess it from the documentation.
Any chance you can upload the generated HTML file somewhere? (You should be able to see the path in the browser address bar on the page with the "Add Cluster As Release..." button.)
Also, what version of the "Add Cluster As Release..." and what version of Picard are you using? Can you provide the logs Picard generates?