I receive a null Gdk::Window with get_window()

[Eduardo Paz <edupaz2 gmail com>]

Hi everyone! I'm new in using gtkmm, and I have a lot of questions. I
know it would be easy but I couldn find the solution looking at the
API.
Well, the first one is how i can color the background of a box (for
example). Here is the code I've written :
// Constructor of Gtk::Window
....
add(box);
box.get_window()->set_bakground(..color..);
...
I got a Sig-sev because the Gdk::Window that get_window returns is NULL.
Is another way to make this or I'm making something wrong??