Why does TypeError: Cannot read property '...' of undefined occur?

Question

Qt

In general terms, I have a FlowChart element whose p1 and p2 properties need to be controlled from the SensorValuesSetter element using text fields.
The implementation of the SensorValuesSetter element is in another file.

// SensorValuesSetter.qml
ApplicationWindow {
        id: valueSetterWindow

        // ...

        GridLayout {
            // ...

            Label { text: "Давление 1: "; Layout.fillWidth:true; }
            ValueInputField { id: p1_val_field; }   

            Label { text: "Давление 2: "; Layout.fillWidth:true; }
            ValueInputField { id: p2_val_field; }   

            // ....

            Button {
                id: button
                text: qsTr("Применить")
                onPressed: {
                    valueSetterWindow.parent.p1.value = Number.fromLocaleString(p1_val_field.text)
                    valueSetterWindow.parent.p2.value = Number.fromLocaleString(p2_val_field.text)
                    // ...
        }
}


// main.qml

// ...

FlowChart {  
        id: flowChart
        anchors.fill: parent


        SensorValuesSetter {
            id: valueSetterWindow
        }

        // ...
    
}

However, when filling one of the values, an error occurs: TypeError: Cannot read property '[property]' of undefined
Please tell me why this error occurs and how to overcome it?