Left-click on plasmoid in system tray fails to execute command

lrnn

Hello and thanks for the help.
If this forum is not for beginners, please point me to a beginners forum. (I'm learning plasmoids and QML.)

I'm trying to create a plasmoid that executes a command when the user left-clicks on its system tray icon.
Unfortunately, I get an error message when I left-click on the plasmoids system tray icon.

"main.qml":

import QtQuick 2.5
import QtQuick.Extras 1.4
import org.kde.plasma.plasmoid 2.0
import org.kde.plasma.core 2.0 as PlasmaCore

Item {
    id: root

    // If the plasmoid in the system tray is left-clicked, then execute a system command.
    Plasmoid.icon: plasmoid.configuration.icon
    Plasmoid.icon.onClicked: { systemCommand.exec('konsole --noclose -e "echo \"Hello world\""') }

    // Execute a system command.
    PlasmaCore.DataSource {
        id: systemCommand
        engine: "executable"
        connectedSources: []
                onNewData: {
                    exited(sourceName)
                    disconnectSource(sourceName) // cmd finished
                }
        function exec(cmd) { connectSource(cmd) }
        signal exited(string sourceName)
    }
}

Error message:

Error loading QML file: file:///home/test/.local/share/plasma/plasmoids/test/contents/ui/main.qml:11:19: Cannot assign a value directly to a grouped property

I've misconfigured "Plasmoid.icon.onClicked", but I don't understand what I've done wrong (still learning object programming).
I'd appreciate any pointers on what I've done wrong.
Thanks for the help.