Casting GTKWidgets - Urgent

[Naheed Vora <nv linuxstart com>]

void
on_combo_entry1_activate               (GtkEditable     *editable,
                                        gpointer         user_data)
{

GtkWidget *text_entry = user_data;
char *string = gtk_entry_get_text(GTK_ENTRY (text_entry));
g_print(string);

}

Error encountered :-
----------------------------------------------------------

Gtk-WARNING **: invalid cast from (NULL) pointer to `GtkEntry'

Gtk-CRITICAL **: file gtkentry.c: line 496 (gtk_entry_get_text):
assertion `entry != NULL' failed.

GLib-CRITICAL **: file gmessages.c: line 645 (g_print): assertion
`format != NULL' failed.

----------------------------------------------------------
Please guide me how can I cast the gpointer into char* which is recieved
from signal caused by Gtk_entry widget.

TIA
nv