Fix save issue when setting select-item in nuxeo-directory-suggestion

Steps to reproduce:

1. Select Nuxeo LTS 2021 (or 2019) and Nuxeo Web UI in Application Definition.
2. In Designer, scaffold the create layout for File.
3. Select the <nuxeo-directory-suggestion> element scaffolded for dc:nature.
4. Set the selected-item property with the value {{document.properties.dc:nature}}.
5. Try to save. The progress bar will spin indefinitely and no error or warning is displayed in the browser console.

This is specially problematic if the change to the layout is done externally. At that point no changes to the layout can be saved unless the property is removed.

Whenever Designer fails to save a layout it should try to fail gracefully.

(reported by Julian Sabos in Slack)