Unable to open tasks that contain subforms on inputs.

Everytime you try to open a task containing a data object subform in data inputs, you get the following error:

Unable to complete your request. The following exception occurred: com.thoughtworks.xstream.security.ForbiddenClassException:${name-of-the-data-object}

This exception prevents the form from rendering.

Steps to reproduce:
1. Open authoring. Import reproducer from: https://github.com/jsoltes/reproducers/tree/master/JBPM_6456_reproducer
2. Click build&deploy.
3. Go to process definitions, start process.
4. Go to task lists, click on the started task.