An error occurred when registering a C++ class using QML_ELEMENT: QQmlApplicationEngine failed to load component

I wrote a simple example while learning QML and CMake. The main purpose is to demonstrate how to use `qt_add_qml_module` and how to register C++ classes in QML.

the key code is as follows:

**//CMakeLists.txt**
add_subdirectory(mybackend)

qt_add_executable(appMyMain
        main.cpp
)

qt_add_qml_module(appMyMain
        URI "MainApp"
        RESOURCE_PREFIX /mycompany/mainapp
        VERSION 1.0
        QML_FILES Main.qml
)

target_link_libraries(appMyMain PRIVATE Qt6::Quick BackendLib)

qt_finalize_executable(appMyMain)

**//main.cpp**
int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    engine.addImportPath(":/mycompany/imports");

    const QUrl url(u"qrc:/mycompany/mainapp/Main.qml"_qs);

    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);

    engine.load(url);

    return app.exec();
}

**//Main.qml**
import QtQuick
import QtQuick.Controls
import MyCompany.Backend

**//mybackend/CMakeLists.txt**

qt_add_library(BackendLib STATIC)
set_target_properties(BackendLib PROPERTIES AUTOMOC ON)
qt_add_qml_module(BackendLib
        URI "MyCompany.Backend"  
        RESOURCE_PREFIX /mycompany/mainapp
        VERSION 1.0
        SOURCES
        MyBackend.h
        MyBackend.cpp
)

target_link_libraries(BackendLib PRIVATE Qt6::Quick)

**//mybackend/Mybackend.h**
#include <QObject>
#include <QtQml>

class MyBackend : public QObject
{
    Q_OBJECT
    QML_ELEMENT
    Q_PROPERTY(QString userName READ userName WRITE setUserName NOTIFY userNameChanged)

Then I received the following error message:

QQmlComponent: attempted to load via a relative URL 'qrc:qt/qml/mycompany/mainapp/Main.qml' in resource file system. This is not fully supported and may not work
QQmlApplicationEngine failed to load component
qrc:qt/qml/mycompany/mainapp/Main.qml: No such file or directory

I have already asked the AI, but it didn't provide me with any effective solutions.