Description:
After upgrading Zabbix from a previous version 7.0.19 to 7.0.20, PHP errors started appearing in the web interface when displaying an SVG Graph widget on a dashboard.

Errors:
Undefined array key "key_" [zabbix.php:17 → require_once() → ZBase->run() → ZBase->processRequest() → CController->run() → Widgets\SvgGraph\Actions\WidgetView->doAction() → Widgets\SvgGraph\Includes\CSvgGraphHelper::get() → CGraphHelper::calculateMetricsDelay() → CGraphHelper::getItemMaxDelay() in include/classes/helpers/CGraphHelper.php:302]

preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated [zabbix.php:17 → require_once() → ZBase->run() → ZBase->processRequest() → CController->run() → Widgets\SvgGraph\Actions\WidgetView->doAction() → Widgets\SvgGraph\Includes\CSvgGraphHelper::get() → CGraphHelper::calculateMetricsDelay() → CGraphHelper::getItemMaxDelay() → preg_match() in include/classes/helpers/CGraphHelper.php:302]

Steps to reproduce:
Open a dashboard containing an SVG Graph widget. The error appears immediately upon page load.

Expected behavior:
The widget should render without errors, as it works on previous version 7.0.19