Can't get text from entry

[César Leonardo Blum Silveira <cesarbs brturbo com>]

Why doesn't the following piece of code work?

#include <gtk/gtk.h>

char *string=NULL;

void combo_change(GtkWidget *widget, gpointer data)
{
   gchar *selected;

   selected = gtk_entry_get_text(GTK_ENTRY(GTK_COMBO(widget)->entry));

   strcpy(string, selected);

   printf("%s\n",string);
}

int main(int argc, char *argv[])
{
   GtkWidget *window;
   GtkWidget *combo;
   GList *opt;

   gtk_init(&argc, &argv);

   window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  
   combo = gtk_combo_new();
   opt = g_list_append(opt, "string 1");
   opt = g_list_append(opt, "string2");
   gtk_combo_set_popdown_strings(GTK_COMBO(combo),opt);

   
g_signal_connect(G_OBJECT(GTK_COMBO(combo)->entry),"activated",G_CALLBACK(combo_change),NULL);

   gtk_container_add(GTK_CONTAINER(window),combo);

   gtk_widget_show_all(window);

   gtk_main();

   return 0;
}

Thanks