Hidden undefined index in trigger expression popup

1. Go to Configuration -> Hosts -> Triggers list.
2. Open existing trigger in edit more.
3. Press "Add" near expression textarea.
4. Observe the hidden error.

Undefined index: A [zabbix.php:21 -> require_once() -> ZBase->run() -> ZBase->processRequest() -> CView->getOutput() -> include() -> CConfigFile->{closure}() in app\views\popup.triggerexpr.php:107]