DesignStudio built with Qt6 fails to read properties of type var

Design Studio built with Qt6 throws an InvalidArgumentException when opening a scene that contains keyframes.

Exception: "InvalidArgumentException"
Function: "setValue"
File: "...src/plugins/qmldesigner/designercore/model/variantproperty.cpp"
Line: 62
"file: .../qt-creator/src/plugins/qmldesigner/designercore/model/variantproperty.cpp, function: setValue, line: 62"

The reason is a failed attempt to convert the value argument which is of type var in propertyparser.cpp::read(int variantType, const QString &str)
...
// with variantType == QVariant::QVariant

value = QVariant(str);
// value == QVariant(QString, "540")

value.convert(static_cast<QVariant::Type>(variantType));
// value == QVariant(QVariant, QVariant(Invalid))

A quickfix that worked on my side was to add a switch case for QVariant that deals with the issue.
Not sure if this should be better fixed in QVariant though. (... or if this an intended behaviour change)