Cannot override attributes/implement handlers in Qt QML form implementation - qt

Question

I'm trying to learn Qt and wrap my head around the UI forms. The doc states that I should be able to implement headers in the implementation file. However, I cannot do this (nor override any attributes assigned in the designer).
Here's my BasicWindowForm.ui.qml:
import QtQuick 2.4
import QtQuick.Controls 2.12
Item {
id: element
width: 400
height: 400
property alias quitButton: quitButton
Rectangle {
id: rectangle
x: 100
width: 200
height: 86
color: "#ffffff"
radius: 10
border.width: 3
anchors.horizontalCenter: parent.horizontalCenter
anchors.top: parent.top
anchors.topMargin: 25
}
Row {
id: row
y: 222
height: 100
layoutDirection: Qt.RightToLeft
anchors.right: parent.right
anchors.rightMargin: 0
anchors.left: parent.left
anchors.leftMargin: 0
anchors.bottom: parent.bottom
anchors.bottomMargin: 0
Button {
id: quitButton
text: qsTr("Quit")
}
}
}
/*##^##
Designer {
D{i:1;anchors_y:20}D{i:2;anchors_width:200;anchors_x:21}
}
##^##*/
Here's how it looks in the design mode:
Here's BasicWindow.qml:
import QtQuick 2.4
BasicWindowForm {
quitButton.text: "Lorem"
quitButton.onClicked: Qt.quit()
}
And main.qml:
import QtQuick 2.12
import QtQuick.Controls 2.12
ApplicationWindow {
id: applicationWindow
visible: true
width: 640
height: 480
title: qsTr("Play QtQuick")
BasicWindowForm {
anchors.fill: parent
id: winform
}
}
If I move the handler to the main.qml file, it works. But, as it is here, clicking on the button does nothing, and it still reads "quit".
What am I doing wrong?