QML Native ScrollBar: Unable to assign [undefined] to int

Working from QDS, there is a qml file in HelperWidgets named VerticallScrollBar.qml. It extends the QtQuickControls / Native ScrollBar.

I get the following error on application output when I hover in the app over the scroll bar: 

qrc:/qt-project.org/imports/QtQuick/Controls/Windows/ScrollBar.qml:69:49: Unable to assign [undefined] to int

which appears to refer to the following line inside quickcontrols2/windows/ScrollBar.qml:

visible: contentItem instanceof NativeStyle.StyleItem
Behavior on opacity { NumberAnimation { duration: contentItem.transitionDuration } }     // <---------- this line

The contentItem doesn't have the "transitionDuration" property, and that is because the contentItem is a Rectangle, rather than a NativeStyle.StyleItem. And that's because VerticalScrollBar.qml above  has the contentItem redefined like this:

contentItem: Rectangle {
    implicitWidth: StudioTheme.Values.scrollBarThickness
    color: StudioTheme.Values.themeScrollBarHandle
}

This is something that the Qt Quick ScrollBar should have supported (given the  "contentItem instanceof NativeStyle.StyleItem" above), but somehow it seems that this animation is being called, and this NativeStyle.ScrollBar object ends up being visible, although the contentItem is redefined as a Rectangle.

Example project attached.

When hovering scrollbar with mouse, you get output
"qrc:/qt-project.org/imports/QtQuick/Controls/Windows/ScrollBar.qml:69:49: Unable to assign [undefined] to int"