Use the properties `valueRole` and `currentValue`, introduced in Qt5.14 for `QQC2.ComboBox`
This didn't work before due to QQC2 version being mis-detected at build time, so the code mocked an internal `currentValue` property.
After D28859, it is now required to fix:
"file:///home/nate/kde/usr/share/kpackage/kcms/kcm_kwinrules/contents/ui/RulesEditor.qml"
"Error loading QML file.\n42: Type RuleItemDelegate unavailable\n68: Type OptionsComboBox unavailable\n35: Cannot override FINAL property\n"
QCoreApplication::postEvent: Unexpected null receiver