Exposing C++ object to QML - the right way

I have a trouble to expose my C++ class to QML. So far I didn't find some description in the Qt documentation  how to do that right, especially the type of the exposed object. The common C++ way to pass a const reference but that doesn't work for me.

Talking in detail, I have an object (Provider) that I exposed to QML using context property. That works fine. The object has a signal that passes another object (Message) that I've registered using `qmlRegisterUncreatableType`. But what should be a signature of the signal? So far I've tried:

void messageReceived(const Message &message);

The application crashes inside QMetaTypeInterface::DtorFn getDtor()

void messageReceived(Message &message);

The same as above.

void messageReceived(const Message *message);

I get en error: TypeError: Property 'getText' of object QVariant(Message, ) is not a function

void messageReceived(Message *message);

This is the only option that works. Why? Is that bug? a feature? Actually I prefer not to pass non-const object if I don't want to change it inside. As I said the common C++ way is to pass const reference. Why doesn't that work?

P.S. After changing the Message to be Q_GADGET all that stops to work at all. The message I get in console:

qt.qml.typeregistration: Invalid QML element name "Message"; value type names should begin with a lowercase letter
qrc:/main.qml:15: TypeError: Property 'getText' of object QVariant(Message*, 0x7fffffffb677) is not a function

What? Why that works fine with Q_OBJECT and doesn't work with Q_GADGET? Ok, Ok, changing `Message` to `message` doesn't solve the issue. The error message still:

TypeError: Property 'getText' of object QVariant(message*, 0x7fffffffb677) is not a function

I really don't understand what I do wrong. I guess that works fine in Qt < 6.0 but can't check it now.