[gtkada] Getting draw in a drawable area

[Nacho]

I've to learn gtkada to do a postcript viewer for a university work. 
My main problem is that i can get to paint in a drawable area.
I've made a lot of tries to get this and the only i get are runtime errors.
Can somebody says me why this code have runtime errors?
Thank you and sorry for my english.

with Glib;
with Gdk.Window;
with Gdk.Drawable;
with Gdk.GC;
with Gdk.Font;
with Gtk.Drawing_Area;
with Gtk.Window;

procedure Dibujo is

   Gdkw : Gdk.Window.Gdk_Window;
   GC   : Gdk.GC.Gdk_GC;
   Font : Gdk.Font.Gdk_Font;
   Dibujo : Gtk.Drawing_Area.Gtk_Drawing_Area;
   Ventana : Gtk.Window.Gtk_Window;

   procedure Draw (Drawing : in out Gtk.Drawing_Area.Gtk_Drawing_Area) is
      Gdkw : Gdk.Window.Gdk_Window;
      GC   : Gdk.GC.Gdk_GC;
      Font : Gdk.Font.Gdk_Font;
      use type Glib.Gint;

   begin
      -- Get the Gdk window

      Gdkw := Gtk.Drawing_Area.Get_Window (Drawing) ;

      -- Clear the window

      GDK.Window.Clear (Gdkw) ;

      -- Create a graphic context associated with this window

      Gdk.GC.Gdk_New (GC, Gdkw);

      -- Draw a line in this window

      Gdk.Drawable.Draw_Line
        (Drawable => Gdkw,
         GC => GC,
         X1 =>   0, Y1 =>   0,
         X2 => 100, Y2 => 100);

      -- Draw an arc

      Gdk.Drawable.Draw_Arc
        (Drawable => Gdkw,
         Gc       => gc,
         Filled   => True,
         X        => 100,
         Y        => 100,
         Width    => 200,
         Height   => 100,
         Angle1   => 0 * 64,
         Angle2   => 270 * 64);

      -- Ask for a given font

      Gdk.Font.Load (Font,
                     "-adobe-courier-medium-i-*-*-15-*-*-*-*-*-*-*");
    Gdk.Drawable.Draw_Text
      (Drawable    => Gdkw,
       Font        => Font,
       Gc          => gc,
       X           => 50,
       Y           => 50,
       Text        => "Hello World");
    Gdk.GC.Destroy (GC);
   end Draw;


begin

   Gtk.Window.Gtk_New (Ventana);
   Gtk.Drawing_Area.Gtk_New (Dibujo);
   Gtk.Window.Add (Ventana, Dibujo);
   Gtk.Window.Show_All (Ventana);
   Draw (Dibujo);

end Dibujo;