_setSheetRefs() {
        // Set the active sheet.
        let bookViewsNode = xmlq.findChild(this._node, "bookViews");
        if (!bookViewsNode) {
            bookViewsNode = { name: 'bookViews', attributes: {}, children: [] };
            xmlq.insertInOrder(this._node, bookViewsNode, nodeOrder);
        }

        let workbookViewNode = xmlq.findChild(bookViewsNode, "workbookView");
        if (!workbookViewNode) {
            workbookViewNode = { name: 'workbookView', attributes: {}, children: [] };
            xmlq.appendChild(bookViewsNode, workbookViewNode);
        }

        workbookViewNode.attributes.activeTab = this._sheets.indexOf(this._activeSheet);

        // Set the defined names local sheet indexes.
        const definedNamesNode = xmlq.findChild(this._node, "definedNames");
        if (definedNamesNode) {
            _.forEach(definedNamesNode.children, definedNameNode => {
                if (definedNameNode.localSheet) {
                    definedNameNode.attributes.localSheetId = this._sheets.indexOf(definedNameNode.localSheet);
                }
            });
        }
    }
Set the proper sheet references in the XML.
@returns {undefined}
@private