Change service to manage <front> element in XML files

src/app/services/xml-parsers/structure-xml-parser.service.ts

@@ -35,7 +35,7 @@ export class StructureXmlParserService {
       };
     }
 
-    const frontPages = frontPbs.length === 0 && this.isMarkedAsOrigContent(front)
+    const frontPages = frontPbs.length === 0 && front && this.isMarkedAsOrigContent(front)
       ? [this.parseSinglePage(doc, front, 'page_front', this.frontTagName)]
       : frontPbs.map((pb, idx, arr) => this.parseDocumentPage(doc, pb as HTMLElement, arr[idx + 1] as HTMLElement, this.frontTagName));