create Qml sub modules

AmirCH81

I’m trying to create a QML module named ERP, with a submodule ERP.Singletons that contains a singleton QML file (ColorPalette.qml).
However, when I run the application, I get the following error:

qrc:/qt/qml/ERP/qml/Main.qml:3:1: module "ERP.Singletons" is not installed

project structure:

.
├── CMakeLists.txt
├── CMakeLists.txt.user
├── qml
│   ├── Main.qml
│   └── singleton
│       └── ColorPalette.qml
└── source
    └── main.cpp

CMakeLists.txt

cmake_minimum_required(VERSION 3.16)
project(ERP VERSION 0.1 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(Qt6 REQUIRED COMPONENTS Quick)
qt_standard_project_setup(REQUIRES 6.8)

qt_add_executable(appERP
    source/main.cpp
)

qt_add_qml_module(appERP
    URI ERP
    VERSION 1.0
    QML_FILES
        qml/Main.qml
)

set_source_files_properties(
    qml/singleton/ColorPalette.qml
    PROPERTIES QT_QML_SINGLETON_TYPE TRUE
)

qt_add_qml_module(ERP_Singletons
    URI ERP.Singletons
    VERSION 1.0
    QML_FILES
        qml/singleton/ColorPalette.qml
)

set_target_properties(appERP PROPERTIES
    MACOSX_BUNDLE TRUE
    WIN32_EXECUTABLE TRUE
    MACOSX_BUNDLE_BUNDLE_VERSION ${PROJECT_VERSION}
    MACOSX_BUNDLE_SHORT_VERSION_STRING ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}
)

target_link_libraries(appERP
    PRIVATE Qt6::Quick ERP_Singletons
)

include(GNUInstallDirs)
install(TARGETS appERP
    BUNDLE DESTINATION .
    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
)

QML Files
Main.qml:

import QtQuick
import QtQuick.Controls
import ERP.Singletons

ApplicationWindow {
    width: 640
    height: 480
    visible: true
    title: "ERP"
    color: ColorPalette.background
}

ColorPalatte.qml:

pragma Singleton
import QtQuick

QtObject {
    property color background: "#333333"
    property color text: "#ffffff"
}

Binary Tree:

build/
├── appERP
├── ERP/
│   ├── qml/
│   │   ├── Main.qml
│   │   └── qmldir
│   └── qmldir
├── ERP_Singletons.qmltypes
├── libERP_Singletons.so
├── libERP_Singletonsplugin.so
└── qml/
    └── singleton/
        ├── ColorPalette.qml
        └── qmldir