PanelPicker API is not working

https://developer.ubuntu.com/api/apps/qml/sdk-15.04/Ubuntu.Components.PickerPanel/

I tried to use the code snippet on the above link. When I run it, it says:

file:///home/liuxg/qml/pickerpanel/Main.qml:32: ReferenceError: PickerPanel is not defined

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import QtQuick 2.4
import Ubuntu.Components 1.2
import Ubuntu.Components.Popups 0.1

/*!
    \brief MainView with a Label and Button elements.
*/

MainView {
    // objectName for functional testing purposes (autopilot-qt5)
    objectName: "mainView"

    // Note! applicationName needs to match the "name" field of the click manifest
    applicationName: "pickerpanel.liu-xiao-guo"

    /*
     This property enables the application to change orientation
     when the device is rotated. The default is false.
    */
    //automaticOrientation: true

    width: units.gu(60)
    height: units.gu(85)

    Page {
        title: i18n.tr("pickerpanel")

        Button {
            id: dateButton
            property date date: new Date()
            text: Qt.formatDateTime(date, "yyyy/MMMM")
            onClicked: PickerPanel.openDatePicker(dateButton, "date", "Years|Months")
        }
    }
}