Content field cannot be previewed in an AWM

Step to reproduce:
1) create an app within minute with a Content field
2) create/edit an item, and enter some text changes in the Content field
3) click Preview, the content field is displayed empty (for a new item), or like the last saved value (for an edited item)

Expected: see the unsaved changes

This is due to the displayer and really close to XWIKI-7908 but in another way.
I have tested a similar fix with success.