patch checking for base parent component of Combobox to check if it exists first. This should be refactored out some time.

CURA-9424

resources/qml/Widgets/ComboBox.qml

@@ -18,6 +18,7 @@ ComboBox
     property var defaultTextOnEmptyModel: catalog.i18nc("@label", "No items to select from")  // Text displayed in the combobox when the model is empty
     property var defaultTextOnEmptyIndex: ""  // Text displayed in the combobox when the model has items but no item is selected
     property alias textFormat: contentLabel.textFormat
+    property alias backgroundColor: background.color
 
     enabled: delegateModel.count > 0
 
@@ -45,7 +46,7 @@ ComboBox
         State
         {
             name: "highlighted"
-            when: (base.hovered || control.hovered) && !control.activeFocus
+            when: ((base != undefined && base.hovered) || control.hovered) && !control.activeFocus
             PropertyChanges
             {
                 target: background
@@ -56,6 +57,7 @@ ComboBox
 
     background: UM.UnderlineBackground
     {
+        id: background
         // Rectangle for highlighting when this combobox needs to pulse.
         Rectangle
         {