I have simple testing code as follow:

test.hpp

#pragma once
#include <QQmlEngine>

class test_gadget {
    Q_GADGET
    QML_ELEMENT
    Q_PROPERTY(int test MEMBER test)
public:
    int test;
};

class test_object : public QObject {
    Q_OBJECT
    QML_ELEMENT
public:
    test_object(const test_gadget&);
private:
    test_gadget gadget;
};

test.cpp

#include <test.hpp>

test_object::test_object(const test_gadget& other) {
    this->gadget = other;
}

main.cpp

#include <QQmlContext>
#include <test.hpp>

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    const QUrl url(u"qrc:/testqt/Main.qml"_qs);
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreationFailed,
        &app, []() { QCoreApplication::exit(-1); },
        Qt::QueuedConnection);
    test_gadget gadget;
    test_object test_obj(gadget);
    engine.rootContext()->setContextProperty("test_obj", &test_obj);
    engine.load(url);

    return app.exec();
}

However when I try to compile, there were errors:

[  5%] Built target apptestqt_qmlimportscan
[ 10%] Built target apptestqt_tooling
[ 15%] Automatic MOC and UIC for target apptestqt
[ 20%] Built target apptestqt_autogen
[ 25%] Running AUTOMOC file extraction for target apptestqt
[ 25%] Built target apptestqt_automoc_json_extraction
[ 30%] Automatic QML type registration for target apptestqt
Error 5 while parsing C:/Users/USER/source/repos/build-testqt-Desktop_Qt_6_6_0_MinGW_64_bit-Debug/meta_types/qt6apptestqt_debug_metatypes.json: illegal value
mingw32-make.exe[2]: *** [CMakeFilesapptestqt.dirbuild.make:88: apptestqt_qmltyperegistrations.cpp] Error 1
mingw32-make.exe[1]: *** [CMakeFilesMakefile2:97: CMakeFiles/apptestqt.dir/all] Error 2
mingw32-make.exe: *** [Makefile:135: all] Error 2
13:22:22: The process "C:QtToolsCMake_64bincmake.exe" exited with code 2.
Error while building/deploying project testqt (kit: Desktop Qt 6.6.0 MinGW 64-bit)
When executing step "Build"

Note: build-testqt-Desktop_Qt_6_6_0_MinGW_64_bit-Debugmeta_typesqt6apptestqt_debug_metatypes.json is empty

How should I fix this?

Qt version: 6.6.0