//before    
background: Rectangle {
        width: control.width
        height: control.height
        color: "transparent"
        border.color: "#e6e6e6"
        border.width: 1
    }

//after    
background: Rectangle {
         id: background // the same property's name
        width: control.width
        height: control.height
        color: "transparent"
        border.color: "#e6e6e6"
        border.width: 1
    }