OK, figured which plasmoid this is about, the standalone yr.no based one.
Seems qt5-declarative changed with Qt 5.12, see file listing for 5.11.2:
:pf XmlListModel
main/qt5-declarative 5.11.2-1 /usr/lib/qt5/qml/QtQuick/XmlListModel/libqmlxmllistmodelplugin.so
main/qt5-declarative 5.11.2-1 /usr/lib/qt5/qml/QtQuick/XmlListModel/plugins.qmltypes
main/qt5-declarative 5.11.2-1 /usr/lib/qt5/qml/QtQuick/XmlListModel/qmldir
main/qt5-declarative-debug 5.11.2-1 /usr/lib/debug/usr/lib/qt5/qml/QtQuick/XmlListModel/libqmlxmllistmodelplugin.so.debug
main/qt5-doc 5.11.2-1 /usr/share/doc/qt5/qtquick/images/qml-xmllistmodel-example.png
main/qt5-doc 5.11.2-1 /usr/share/doc/qt5/qtquick/qml-qtquick-xmllistmodel-xmllistmodel-members.html
main/qt5-doc 5.11.2-1 /usr/share/doc/qt5/qtquick/qml-qtquick-xmllistmodel-xmllistmodel.html
main/qt5-doc 5.11.2-1 /usr/share/doc/qt5/qtquick/qml-qtquick-xmllistmodel-xmlrole-members.html
main/qt5-doc 5.11.2-1 /usr/share/doc/qt5/qtquick/qml-qtquick-xmllistmodel-xmlrole.html
main/qt5-doc 5.11.2-1 /usr/share/doc/qt5/qtquick/qtquick-xmllistmodel-qmlmodule.html
No results for that :pf
command with 5.12. If Qt no longer ships it, than it is an upstream issue, they will need to adjust the plasmoid for this change.