same issue

after doing `pkcon refresh && pkcon update` discover opened but with these :

~$ plasma-discover
invalid kns backend! "/usr/share/knsrcfiles/ksysguard.knsrc" because: "Config group not found! Check your KNS3 installation."
adding empty sources model QStandardItemModel(0x5594f2b07640)
no packages for "org.kde.plasma.quicklaunch"
no packages for "org.kde.plasma.systemloadviewer"
no packages for "org.kde.plasma.showActivityManager"
no packages for "org.kde.plasma.grouping"
no packages for "org.kde.plasma.diskquota"
no packages for "org.kde.plasma.timer"
no packages for "org.kde.kscreen"
no packages for "org.kde.plasma.appmenu"
no packages for "org.kde.plasma.kimpanel"
no packages for "org.cockpit_project.cockpit"
no packages for "im.riot.webapp"
file:///usr/lib/x86_64-linux-gnu/qt5/qml/org/kde/kirigami.2/ScrollablePage.qml:187: TypeError: Cannot assign to read-only property "parent"
file:///usr/lib/x86_64-linux-gnu/qt5/qml/org/kde/kirigami.2/ScrollablePage.qml:187: TypeError: Cannot assign to read-only property "parent"
file:///usr/lib/x86_64-linux-gnu/qt5/qml/org/kde/kirigami.2/ScrollablePage.qml:187: TypeError: Cannot assign to read-only property "parent"
file:///usr/lib/x86_64-linux-gnu/qt5/qml/org/kde/kirigami.2/ScrollablePage.qml:187: TypeError: Cannot assign to read-only property "parent"
no packages for "org.kde.plasma.quicklaunch"
no packages for "org.kde.plasma.systemloadviewer"
no packages for "org.kde.plasma.showActivityManager"
no packages for "org.kde.plasma.grouping"
no packages for "org.kde.plasma.diskquota"
no packages for "org.kde.plasma.timer"
no packages for "org.kde.kscreen"
no packages for "org.kde.plasma.appmenu"
no packages for "org.kde.plasma.kimpanel"
no packages for "org.cockpit_project.cockpit"
no packages for "im.riot.webapp"
file:///usr/lib/x86_64-linux-gnu/qt5/qml/org/kde/kirigami.2/templates/private/ScrollView.qml:95:5: QML QQuickItem: ScrollBar must be attached to a Flickable or ScrollView
qrc:/qml/ApplicationsListPage.qml:144:16: QML ResourcesProxyModel: Binding loop detected for property "sortOrder"
QObject: Cannot create children for a parent that is in a different thread.
(Parent is QQmlApplicationEngine(0x5594f10cbc70), parent's thread is QThread(0x5594f10131b0), current thread is QSGRenderThread(0x5594f3414b20)

FWIW: I split this topic, as asking in a post over a year old about a completely different release is certainly not a good idea...

So what is your exact Neon release and Plasma version? See KInfoCenter for this.