QDesktopServices::openUrl() fails on KDE Plasma

The following test application fails on Kubuntu 18.04

#include <QGuiApplication>
#include <QDesktopServices>
#include <QUrl>

int main(int argc, char *argv[])
{
    QGuiApplication a(argc, argv);
    QDesktopServices::openUrl(QUrl("https://www.qt.io"));
}

with the following error message:

Unable to detect a web browser to launch 'https://www.qt.io'

The URLs in KDE applications' About dialogs, the links in the Welcome screen of Qt Creator produce the above error message when run from a terminal.

The same test application works as expected on macOS with Qt 5.9.5.