After upgrading Zabbix from version 3.4 to 4.0 one classic graph in a dashboard displays with error "Undefined index: name [zabbix.php:21 → require_once() → ZBase->run() → ZBase->processRequest() → CController->run() → CControllerWidgetGraphView->doAction() in app/controllers/CControllerWidgetGraphView.php:263]". 

The actual graph displayed differs from that specified in the dashboard settings. The other graphs on the dashboard are displayed without any errors. 

additional info:

1. Admin user doesn't experience this problem.
2. Tried from different machines.
3. Graph is displayed normally if viewed from Monitoring -> Graphs.