Dissallow empty Div creation in SME

[Dananji]

Description

SME renders the structure from a nested list of Ranges under structures property in a IIIF manifest.

According to the Presentation 3.0 spec, a Range must have an items property with at least one item (https://iiif.io/api/presentation/3.0/#34-structural-properties).

To abide this when creating/changing structures in SME, we should disallow saving empty Div (heading) items.

Done Looks Like

• Do not allow saving structure with empty heading items (Divs)
• Mark structure heading items without children when using erroneous manifests (for external uses in the demo site, not specifically in Avalon)

Definition of Done

• SME disallows saving structure with heading items (Divs) without children in Avalon
• In the public facing demo site: opening a manifest where the structure has empty heading items (Divs) display them as invalid items.

[elynema]

Potentially the Avalon endpoint for saving structure could return error message if there is invalid data, and that could be returned to both XML editor and SME. We do have an XSD that could be used for validation.