Recently this error showed up on my Kubuntu 23.10 while trying to run applications using QtQuick, such as QtDesignStudio or just a new QML project with QtCreator. The application either outright does not run or has serious GUI issues.

Turns out this is probably an issue related to software and distributions switching from Qt5 to Qt6.

The org.kde.desktop module seems to be a “desktop style” module from KDE. This module is indeed installed on Kubuntu by default:

qml-module-org-kde-qqc2desktopstyle/mantic,now 5.110.0-0ubuntu1 amd64 [installed,automatic]
  Qt Quick Controls 2: Desktop Style

However this is a Qt5 version. Applications built on Qt6 seem to try to locate its Qt6 version and fail. There does not seem to be a Qt6 package in the Ubuntu repositories.

I tried building the Qt6 version from source but it requires more missing Qt6 dependencies.

However, it’s possible to use a workaround – choose a different QtQuick style that is installed. You can either do that using an environment variable:

QT_QUICK_CONTROLS_STYLE=Basic

Or inside the application’s main.cpp itself, right after creating QGuiApplication instance:

QQuickStyle::setStyle(QStringLiteral("Basic"));

That’s it. This issue does not show up in KDE Neon with Plasma 6 (running Qt6). Plasma 6 should be included in the upcoming Kubuntu 24.10.