Handle errors while generating schema.org metadata

* this can happen for drafts, e.g. when "date" metadata fields aren't
  filled out which crashes with a `KeyError`

invenio_theme_tuw/views.py

@@ -134,7 +134,11 @@ def create_blueprint(app):
     @blueprint.app_template_global("tuw_create_schemaorg_metadata")
     def tuw_create_schemaorg_metadata(record):
         """Create schema.org metadata to include in a <script> tag."""
-        return SchemaorgJSONLDSerializer().serialize_object(record)
+        try:
+            return SchemaorgJSONLDSerializer().serialize_object(record)
+        except Exception as e:
+            current_app.logger.error(e)
+            return '{"@context": "http://schema.org"}'
 
     @blueprint.app_template_global("record_count")
     def record_count():