MessageDialog QML Type does not accept common properties

[ars1614]

Hi everybody!
I am using this type for generating user confirming actions.
This is the code I am writing:

MessageDialog {
        id: messageDialog
        property string titleStr: ''
        property string textStr: ''

        title: titleStr
        text: textStr
        standardButtons: StandardButton.Yes | StandardButton.No
        icon: StandardIcon.Question

        onYes: {
            ...
        }

        onNo: {
            ...
        }
} 

It is not accepting the properties: title, text, standartButtons... they are in red with (M16) error, but it compiles and I can run it.

Any reason?