Steps to reproduce:

1. Create a dummy host and assign for example icmp ping template to it
2. Create a new map with 2 elements, for example, the dummy host created in step 1 and some image.
3. In this map connect the 2 elements together and assign a link indicator trigger - icmp unavailable from dummy host. Save map.
4. Use some existing/create a new dashboard with Map navigation tree, add the map created in step 2-3
5. Disable icmp unavailable trigger in dummy host triggers.
6. Try to open/reload the dashboard containing the map navigation tree used in step 4

Result:
Map navigation tree does no longer load and also the associated map element does no longer show anything. 
Browser trace shows HTTP Error 500 to the request to zabbix.php?sid=e3719d66c64614d7&action=widget.navtree.view

In web server error logs:

PHP Notice:  Undefined offset: 27606 in /usr/share/zabbix/app/controllers/CControllerWidgetNavTreeView.php on line 251, referer: https://myzabbixserver/zabbix.php?action=dashboard.view

and after that:
PHP Fatal error:  Uncaught TypeError: Argument 2 passed to CControllerWidgetNavTreeView::sumArrayValues() must be of the type array, null given, called in /usr/share/zabbix/app/controllers/CControllerWidgetNavTreeView.php on line 263 and defined in /usr/share/zabbix/app/controllers/CControllerWidgetNavTreeView.php:507\nStack trace:\n#0 /usr/share/zabbix/app/controllers/CControllerWidgetNavTreeView.php(263): CControllerWidgetNavTreeView::sumArrayValues(Array, NULL)\n#1 /usr/share/zabbix/app/controllers/CControllerWidgetNavTreeView.php(482): CControllerWidgetNavTreeView->getNumberOfProblemsBySysmap(Array)\n#2 /usr/share/zabbix/include/classes/mvc/CController.php(472): CControllerWidgetNavTreeView->doAction()\n#3 /usr/share/zabbix/include/classes/core/ZBase.php(530): CController->run()\n#4 /usr/share/zabbix/include/classes/core/ZBase.php(210): ZBase->processRequest(Object(CRouter))\n#5 /usr/share/zabbix/include/config.inc.php(25): ZBase->run('default')\n#6 /usr/share/zabbix/zabbix.php(22): require_once('/usr/share/zabb...')\n#7 {main}\n  t in /usr/share/zabbix/app/controllers/CControllerWidgetNavTreeView.php on line 507, referer: https://myzabbixserver/zabbix.php?action=dashboard.view

Expected:
Man navigation tree should work

P.S. this seems to have been broken somewhere after v 6.0.5. As on 6.0.5 this worked without any issues. 
P.P.S. also tried to manually update the two changed php files from ZBX-21562, but the issue still seems to persist.