SUMMARY
starting kcmshell5 kcm_kwin_virtualdesktops
shows a ton of errors:

file:///usr/lib64/qt5/qml/org/kde/kirigami.2/FormLayout.qml:137:9: Unable to assign [undefined] to double
file:///usr/lib64/qt5/qml/org/kde/kirigami.2/FormLayout.qml:137:9: Unable to assign [undefined] to double
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:66: TypeError: Cannot read property 'display' of null
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:89: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:79: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:66: TypeError: Cannot read property 'display' of null
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:89: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:79: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:66: TypeError: Cannot read property 'display' of null
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:89: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:79: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:66: TypeError: Cannot read property 'display' of null
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:89: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:79: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:66: TypeError: Cannot read property 'display' of null
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:89: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:79: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:66: TypeError: Cannot read property 'display' of null
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:89: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:79: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:66: TypeError: Cannot read property 'display' of null
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:89: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:79: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:66: TypeError: Cannot read property 'display' of null
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:89: TypeError: Value is null and could not be converted to an object
file:///usr/share/kpackage/kcms/kcm_kwin_virtualdesktops/contents/ui/main.qml:79: TypeError: Value is null and could not be converted to an object
Constructing a KPluginInfo object from old style JSON. Please use kcoreaddons_desktop_to_json() for "/usr/lib64/qt5/plugins/kwin/effects/configs/kcm_kwin4_genericscripted.so" instead of kservice_desktop_to_json() in your CMake code.