Don't display the Participating documents form on the workflow model

On the workflow model, a form is available that enables to add participating documents. But clicking on the Excute button throws an error.

Steps to reproduce:

1. As an Administrator, go on the Workflow tab of the Admin Center.
2. Click on a workflow.
3. Click on the Manage tab.
   -> A Content subtab is displayed.
4. At the bottom of the tab, a Participating documents form is displayed.
5. Search for a document and select it.
6. Click on the Execute button.
   => An error occurs.