It completely works

app_win_2_back.cpp

@@ -0,0 +1,61 @@
+#include "app_win_2_back.hpp"
+#include "hello_world.hpp"
+#include <iostream>
+
+namespace gui_to_app {
+    AppWin2Back::AppWin2Back() : Gtk::Application("org.billy_adventures.character_sheet", Flags::HANDLES_OPEN) {
+        Glib::set_application_name("Billy's character sheet");
+    }
+
+    void AppWin2Back::on_startup() {
+        Application::on_startup();
+        const auto menu_bar = []() -> Glib::RefPtr<Gio::Menu> {
+            try {
+                const auto builder = Gtk::Builder::create_from_file("menu_bar.ui");
+                return builder->get_object<Gio::Menu>("menu");
+            } catch (const std::exception &e) {
+                std::cerr << e.what() << '\n';
+                return nullptr;
+            }
+        }();
+        if (!menu_bar) {
+            std::cerr << "Error occured while loading menu bar's.\n";
+        } else {
+            set_menubar(menu_bar);
+            add_action("quit", sigc::mem_fun(*this, &AppWin2Back::quit));
+            set_accel_for_action("app.quit", "<Ctrl>q");
+        }
+    }
+
+    Glib::RefPtr<AppWin2Back> AppWin2Back::create() {
+        return Glib::make_refptr_for_instance(new AppWin2Back());
+    }
+
+    void AppWin2Back::on_activate() {
+        Application::on_activate();
+
+        const auto app_builder = [this]() -> Glib::RefPtr<Gtk::Builder> {
+            try {
+                return Gtk::Builder::create_from_file("window_ui.ui");
+            } catch (const std::exception &e) {
+                std::cerr << e.what() << std::endl;
+                quit();
+                return nullptr;
+            }
+        }();
+        if (!app_builder) {
+            std::cerr << "Error while loading UI.\n";
+            quit();
+            return;
+        }
+        main_window = Gtk::Builder::get_widget_derived<learn_gtkmm4::HelloWorld>(app_builder, "main_window");
+        if (main_window != nullptr) {
+            add_window(*main_window);
+            main_window->set_show_menubar(true);
+            main_window->set_visible(true);
+        } else {
+            quit();
+        }
+    }
+
+} // gui_to_app