assertion failure when using children() to get objectlist

Trying to get a list of children using children() but results into an assert
`
ASSERT: "!isEmpty()" in file ../../../../Qt5.4.1/5.4/gcc/include/QtCore/qlist.h, line 293
`
I've narrowed it down to following example

import QtQuick 2.4

Rectangle {
    width: 100
    height: 100

    property int xval: 0

    Behavior on xval {
        SmoothedAnimation {
            //id: a
            duration: 800
        }
    }

    /*Rectangle {
        width: 50
        height: 50

        Rectangle {
            width: 10
            height: 10
        }
    }*/
}

and loading it as

QQuickView view;
view.setSource(QStringLiteral("qrc:/main.qml"));
view.show();

QQuickItem *item = view.rootObject();
qDebug() << item->children().first()->children().first();

This should actually ouput the SmoothedAnimation object but the children list is actually 0. Now if you specify an id to SmoothedAnimation, the object is found. Is there any compulsion that an id is required ?
I tried the similar scenario with Rectangle (commented code) and it works as expected.