This bug results from QTBUG-108612.

QtWebEngine uses foreign types in form of:

namespace ForeignWebEngineCertificateErrorNamespace
{
    Q_NAMESPACE
    QML_FOREIGN_NAMESPACE(QWebEngineCertificateError)
    QML_NAMED_ELEMENT(WebEngineCertificateError)
    QML_ADDED_IN_VERSION(1, 1)
    QML_EXTRA_VERSION(2, 0)
}

which is used for example like:

  var handleCertificateError = function(error) {
                if (deferError)
                    error.defer()
                else if (acceptCertificate)
                    error.acceptCertificate()
                else
                    error.rejectCertificate()
            }
            view.certificateError.connect(handleCertificateError)

The issue that QWebEngineCertificateError has only a copy constructor , but it is used here as value type. This is not correct as value types should have a default constructor. We do not want to use anonymous qml elements as our classes need proper documentation, therefore qml elements have to stayed named.

Instead we would like to use a value type which is QML_UNCREATABLE which would mean it can not be used as a property and it does not have a default constructor.

property webEngineCertificateError myError

This way we have value proper semantic , with value not instanceable by user , passed (copied) in various signal handlers.