why qml virtualkeyboard is not show after click on edit text ,while I call plugin in .pro and main.cpp

I use open source qt5.9 for an embedded device. I wanna to use virtual keyboard in my qml project. I know I should add a static link in .pro file like :

static {
    QT += svg
    QTPLUGIN += qtvirtualkeyboardplugin
}

and also add

#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    qputenv("QT_IM_MODULE", QByteArray("qtvirtualkeyboard"));

    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;
    return app.exec();
}

to main.cpp file to use the virtual keyboard. but my virtual keyboard does not fire when I click on my text object:

import QtQuick 2.9
import QtQuick.Window 2.2
import QtQuick.VirtualKeyboard 2.1

Window {
    visible: true
    width: 640
    height: 480

        TextInput {
            id: textInput;
            text:"ssssss"
            height: 120;
            width: parent.width - 2;
            anchors.bottom: keyboard.top
            color: "#000000"; // black

            // http://doc.qt.io/qt-5/qinputmethod.html#properties
            focus: Qt.inputMethod.visible;

            verticalAlignment: TextInput.AlignVCenter;
        }
}