QT 6 qmlengine->evaluate('Qt.WindowModal') returns undefined

I am author of Nelson with QML engine available from an numerical interpreter and I try to port it to Qt6.)
https://wiki.qt.io/Language_Bindings (QML bindings for Nelson language)

In Qt5, to convert type "easily", I use:
QJSValue evaluationResult = qmlengine->evaluate('Qt.WindowModal');
and evaluationResult is valid and have as content Qt.WindowModal value.

In Qt6, it returns undefined.
Help would be appreciated.

How convert "generic" string to value
example in nelson language:
```
h.modality = qml_evaluatestring('Qt.WindowModal');
```