Changing Attractor system to null make app crashes

ParticleSytem {
        id: particleSystem
}

Attractor {
        id: attractor
        system: particleSystem
        anchors.fill: parent
        strength: 2

        MouseArea {
            anchors.fill: parent

            onPressed: {
                 //app crash here
                 attractor.system = null
            }
        }
    }