Making sure the window ptr is not null

CMakeLists.txt

@@ -1,9 +1,22 @@
 cmake_minimum_required(VERSION 3.30)
 project(LearnGtk4 LANGUAGES CXX)
 
+include(FetchContent)
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(GTKMM4 REQUIRED IMPORTED_TARGET gtkmm-4.0)
 
+fetchcontent_declare(BillySheet
+        GIT_REPOSITORY https://gitea.pcornatcloud.fr/Pcornat/BillySheet.git
+        GIT_SUBMODULES_RECURSE ON
+        GIT_SHALLOW ON
+        DOWNLOAD_EXTRACT_TIMESTAMP OFF
+        OVERRIDE_FIND_PACKAGE
+)
+option(BILLY_ENABLE_TESTS "Enable Unit testing target" OFF)
+fetchcontent_makeavailable(BillySheet)
+
+find_package(BillySheet CONFIG REQUIRED)
+
 find_program(CCACHE_FOUND ccache)
 if (CCACHE_FOUND)
     message(STATUS "ccache found !")
@@ -86,4 +99,4 @@ target_compile_definitions(LearnGtk4 PUBLIC $<$<CONFIG:Debug>:_GLIBCXX_DEBUG>)
 target_compile_definitions(assimp PUBLIC $<$<CONFIG:Debug>:_GLIBCXX_DEBUG>)
 target_compile_options(LearnGtk4 PUBLIC ${COMPILE_FLAGS})
 target_link_options(LearnGtk4 PUBLIC ${LINKER_OPTIONS})
-target_link_libraries(LearnGtk4 PkgConfig::GTKMM4 assimp spdlog_header_only)
+target_link_libraries(LearnGtk4 PkgConfig::GTKMM4 assimp spdlog_header_only BillySheet)

app_win_2_back.cpp

@@ -112,9 +112,11 @@ namespace gui_to_app {
     void AppWin2Back::on_quit() noexcept {
         auto windows = get_windows();
         for (auto *window: windows) {
+            if (window != nullptr) {
                 window->set_visible(false);
                 delete window;
             }
+        }
         quit();
     }
 } // gui_to_app