Otherwise we leave a context dangling with no engine, which is weird.
I suspect it was done like this as we need the context to outlive the
lifespan of the superclass QQmlObject, but we can use the QObject
child-parent for that.
It shouldn't have any negative behavioural changes as this matches the
behaviour of QmlObject which all original code was once ported from.
See also https://bugreports.qt.io/browse/QTBUG-78326