Empty dropdown list shown on ComboBox after clear

import QtQuick 2.7
import QtQuick.Window 2.2
import QtQuick.Controls 2.0

Window {
    visible: true
    width: 300
    height: 300
    title: qsTr("Hello World")

    ComboBox {
        id: comboBox
        height: 30
        width: 100
        font.pixelSize: 20

        contentItem: Text {
            text: comboBox.displayText
            font: comboBox.font
            color: comboBox.activeFocus ? "black" : "gray"
            horizontalAlignment: Text.AlignLeft
            verticalAlignment: Text.AlignVCenter
        }

        background: Rectangle {
            color: comboBox.activeFocus ? "pink" : "skyblue"
        }

        delegate: ItemDelegate {
            width: comboBox.width
            contentItem: Text {
                text: modelData
                verticalAlignment: Text.AlignVCenter
            }
            highlighted: comboBox.highlightedIndex == index
        }

        model: ListModel {
        }

        onActivated: {
            console.log("clicked: " + currentText + " currentIndex: " + currentIndex)
        }
    }

    Button{
        id: append
        text: "append"
        width: 80
        height: 30
        anchors.left: comboBox.right
        anchors.leftMargin: 3

        property int number: 0

        onClicked: {
            comboBox.model.append({"modelData": number})
            number++

            console.log("count " + comboBox.count);
        }
    }

    Button{
        text: "clear"
        width: 80
        height: 30
        anchors.left: append.right
        anchors.leftMargin: 3

        property int number: 0

        onClicked: {
            comboBox.model.clear()
        }
    }
}