Hmm. your Qt version seems new enough.
if you google "qt qml grabwindow bug" you can see there been quite some issues but
it should work with new Qt versions.

How type of android device are you using ? I mena is it old or kinda new ?

What Qt version are you using?

qDebug() << image.isNull();

qDebug() <<"image.isNull() :" << image.isNull();
return:
image.isNull() : false

foreach(QObject* obj, engine.rootObjects()) {
QQuickWindow* window = qobject_cast<QQuickWindow*>(obj);
if (window) {
QImage image = window->grabWindow();
qDebug() << image;
}
}

QQuickWindow* currentWindow = qobject_cast<QQuickWindow*>(QGuiApplication::focusWindow());

// if(!currentWindow)
// return;
QScreen* screen = currentWindow->screen();
// if(!screen)
// return;
//QRect screenGeometry = fullScreen ? screen->geometry() : currentWindow->geometry();
/* QRect screenGeometry = fullScreen ? screen->geometry() : currentWindow->geometry();
auto pm = screen->grabWindow(0, screenGeometry.x(), screenGeometry.y(), screenGeometry.width(), screenGeometry.height());

QByteArray bArray;
QBuffer buffer(&bArray);
buffer.open(QIODevice::WriteOnly);
pm.save(&buffer, "PNG");*/



QImage myScreen;
//QPixmap pic = QPixmap::grabWindow(QWidget::winId(),0,0,50,50);
//qDebug()<<QWidget::winId();

// QScreen *screen = QGuiApplication::primaryScreen();
QPixmap pixamp;
pixamp = screen->grabWindow(0);
myScreen = pixamp.toImage();
//myScreen = QPixmap::grabWindow();
//myScreen = quickView->grabWindow();
//myScreen.save("C:\Users\New Owner\Downloads\i.png");
QByteArray bArray;
QBuffer buffer(&bArray);
buffer.open(QIODevice::WriteOnly);
//myScreen.save(&buffer, "JPEG");
//QString image("data:image/jpg;base64,");
myScreen.save(&buffer, "PNG");
QString image("data:image/png;base64,");
image.append(QString::fromLatin1(bArray.toBase64().data()));
qDebug()<<"screenShot:"<<image;
return image;

QQuickWindow* currentWindow = qobject_cast<QQuickWindow*>(QGuiApplication::focusWindow());
//    if(!currentWindow)
//        return;
     QScreen* screen = currentWindow->screen();