private function addTrackerChild(Tracker $tracker, SimpleXMLElement $trackers_node)
 {
     $on_top_config = $this->config_factory->getOnTopConfig($tracker);
     if ($on_top_config->isEnabled()) {
         $tracker_node = $trackers_node->addChild(CardwallConfigXml::NODE_TRACKER);
         $tracker_node->addAttribute(CardwallConfigXml::ATTRIBUTE_TRACKER_ID, 'T' . $tracker->getId());
         if (count($on_top_config->getDashboardColumns()) > 0) {
             $columns_node = $tracker_node->addChild(CardwallConfigXml::NODE_COLUMNS);
             foreach ($on_top_config->getDashboardColumns() as $column) {
                 $column_node = $columns_node->addChild(CardwallConfigXml::NODE_COLUMN);
                 $column_node->addAttribute(CardwallConfigXml::ATTRIBUTE_COLUMN_LABEL, $column->getLabel());
             }
         }
     }
 }