Fix run-time null error on Brief page for DC Inputs Icon

Fix that Global.dcInputs.model.firstObject can become null (e.g. if simulator stops)
causing the inputType to become inaccessible.
Use optional chaining to return a null into the Global.dcInputs.inputTypeIcon() function which
returns the default value "qrc:/images/icon_dc_24.svg" in this case.

pages/BriefPage.qml

@@ -14,9 +14,7 @@ SwipeViewPage {
 	property real _gaugeLabelMargin: Theme.geometry_briefPage_edgeGauge_label_initialize_margin
 	property real _gaugeArcOpacity: 0
 	property real _gaugeLabelOpacity: 0
-	readonly property string _dcInputIconSource: Global.dcInputs.model.count === 1
-		 ? Global.dcInputs.inputTypeIcon(Global.dcInputs.model.firstObject.inputType)
-		 : "qrc:/images/icon_dc_24.svg"
+	readonly property string _dcInputIconSource: Global.dcInputs.inputTypeIcon(Global.dcInputs.model.firstObject?.inputType)
 
 	readonly property int _leftGaugeCount: (acInputGauge.active ? 1 : 0) + (dcInputGauge.active ? 1 : 0) + (solarYieldGauge.active ? 1 : 0)
 	readonly property int _rightGaugeCount: dcLoadGauge.active ? 2 : 1  // AC load gauge is always active