Xubuntu 21.10. Почему кнопка на gtk форме не работает?

#include <gtk/gtk.h>
static void activate(GtkApplication *app, void *user_data) {
    GtkWidget *window = gtk_application_window_new(app);
    GtkWidget *win = gtk_window_new ();
    gtk_window_set_application (GTK_WINDOW (win), GTK_APPLICATION (app));
    gtk_window_set_default_size (GTK_WINDOW (win), 400, 300);
    gtk_widget_show (win);

    GtkWidget *lab = gtk_label_new ("Hello.");
    gtk_window_set_child (GTK_WINDOW (win), lab);
    gtk_widget_show (win);


    GtkWidget *box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 5);
    gtk_box_set_homogeneous (GTK_BOX (box), TRUE);
    gtk_window_set_child (GTK_WINDOW (win), box);

    GtkWidget *btn1 = gtk_button_new_with_label ("Hello.");
    g_signal_connect (btn1, "clicked",G_CALLBACK (gtk_window_close), NULL);
    GtkWidget *btn2 = gtk_button_new_with_label ("Quit");
    g_signal_connect (btn2, "clicked", G_CALLBACK (gtk_window_close), win);

    gtk_box_append (GTK_BOX (box), btn1);
    gtk_box_append (GTK_BOX (box), btn2);

    gtk_widget_show (win);

}

int main(int argc, char *argv[]) {
    g_autoptr(GtkApplication) app = gtk_application_new(NULL, G_APPLICATION_FLAGS_NONE);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);

    return g_application_run(G_APPLICATION(app), argc, argv);
}