I need to get the GtkWindow or toplevel widget from my G_DEFINE_TYPE:

MyAppWindow.c:

#include "MyAppWindow.h"

struct _MyAppWindow
{
    GtkApplicationWindow parent;
};

G_DEFINE_TYPE (MyAppWindow, my_app_window, GTK_TYPE_APPLICATION_WINDOW);

void my_app_window_init (MyAppWindow *app)
{
}

void my_app_window_class_init (MyAppWindowClass *class)
{
}

MyAppWindow * my_app_window_new (MyApp *app)
{
    return g_object_new (MY_APP_WINDOW_TYPE, "application", app, NULL);
}

MyAppWindow.h:

#ifndef _MY_APP_WINDOW_H_INCLUDED
#define _MY_APP_WINDOW_H_INCLUDED

#include "MyApp.h"

#define MY_APP_WINDOW_TYPE (my_app_window_get_type ())
G_DECLARE_FINAL_TYPE (MyAppWindow, my_app_window, MY, APP_WINDOW, GtkApplicationWindow)

MyAppWindow * my_app_window_new (MyApp *);

#endif

In my activate prototype:

void my_app_activate (GApplication *app)
{
     MyAppWindow *win = my_app_window_new (MY_APP (app));
     // .. more code
     g_action_map_add_action_entries (G_ACTION_MAP (app), entries, G_N_ELEMENTS (entries), app);
}

In some callback:

void cb_showmsg (GSimpleAction *action, GVariant *parameter, gpointer user_data)
{
     // user_data: need here to access both GApplication and MyAppWindow
}

I need MyAppWindow to show or hide the toplevel window, any ideas?