How can I access gtk_tree_view declared in interface.c from callback.c ?

[Colossus <colossus 3000 it>]

Hi,

I have a file interface.c where glade set up a gtk treeview named 
treeview1. When in callbacks.c I try to use treeview1 I got this error:

callbacks.c: In function `on_new1_activate':
callbacks.c:56: warning: dereferencing `void *' pointer
callbacks.c:56: error: request for member `treeview1' in something not a 
structure or union

This is the code:
void on_new1_activate (GtkMenuItem *menuitem, gpointer user_data)
{
        //qui devo creare un nuovo archivio.    
        gchar *path = Show_File_Dialog ( FALSE );
        if (path == NULL) return;
        g_print ("Hai scelto: %s\n",path);
        switch (ChoosenArchiveType)
        {
                case 0:
                g_print ("Hai scelto: bzip2\n");
                break;
                case 1:
                g_print ("Hai scelto: rar\n");
                break;
                case 2:
                g_print ("Hai scelto: tar\n");
                break;
                case 3:
                g_print ("Hai scelto: zip\n");
                break;
                default:
                break;
        }
        g_free (path);
56:     user_data->treeview1 = NULL; <----- here I got the error
}

Thanks for the attention,
--
Colossus

Cpsed, a Linux OpenGL 3D scene editor
http://cpsed.sourceforge.net/