(Qt5) JSON.stringify call Error. Error message is "Converting circular structure to JSON"

[Bob Wang]

Anybody can help me?
@main.qml

import QtQuick 2.0
import QtQuick.Controls 1.0
import QtQuick.Layouts 1.0
Rectangle {
Button{
id: testButton
text:"Test"
}
Text{
id: testText
text:"TEST"
}

Component.onCompleted: {
    console.log("JSON data = " + JSON.stringify(testText));   //It's OK
    console.log("JSON data = " + JSON.stringify(testButton)); //Get an error "TypeError: Converting circular structure to JSON"
}

}@