QtQuick.Pdf PdfDocument gets "QCoreApplication::postEvent: Unexpected null receiver" warning when instantiated standalone

Standalone reproducer

import QtQuick
import QtQuick.Pdf

Window {
    id: root
    width: 480
    height: 640
    visible: true

    PdfDocument {
        source: "https://dagrs.berkeley.edu/sites/default/files/2020-01/sample.pdf"
    }
}

(Issue does not occur if PdfDocument is replaced with Image)

Outcomes

The document does not load.

The Application Output says:

QCoreApplication::postEvent: Unexpected null receiver
QFSFileEngine::open: No file name specified

If the HTTP(S) URL is replaced with a local URL, then the Application Output says:

QCoreApplication::postEvent: Unexpected null receiver

Original report
Running the attached qml file from the qml tool results in the following error reported to the terminal:

$ qml nullReceiver.qml
QCoreApplication::postEvent: Unexpected null receiver

If the source property is commented out, the error is not reported.

The error occurs apparently with any PDF file.