_parseSheetRefs() {
        // Parse the active sheet.
        const bookViewsNode = xmlq.findChild(this._node, "bookViews");
        const workbookViewNode = bookViewsNode && xmlq.findChild(bookViewsNode, "workbookView");
        const activeTabId = workbookViewNode && workbookViewNode.attributes.activeTab || 0;
        this._activeSheet = this._sheets[activeTabId];

        // Set the location sheet on the defined name nodes. The defined name should point to the index of the sheet
        // but reordering sheets messes this up. So store it on the node and we'll update the index on XML build.
        const definedNamesNode = xmlq.findChild(this._node, "definedNames");
        if (definedNamesNode) {
            _.forEach(definedNamesNode.children, definedNameNode => {
                if (definedNameNode.attributes.hasOwnProperty("localSheetId")) {
                    definedNameNode.localSheet = this._sheets[definedNameNode.attributes.localSheetId];
                }
            });
        }
    }
Parse the sheet references out so we can reorder freely.
@returns {undefined}
@private