[dia] Don't crash on Help, if there is no diagram open

[Hans Breuer <hans src gnome org>]

commit 372dbf356321f2d4fe633a875f70a50fdfef7589
Author: Hans Breuer <hans breuer org>
Date:   Thu Jul 29 20:42:03 2010 +0200

    Don't crash on Help, if there is no diagram open
    
    Also the use of ddisp->menubar was changed because it is NULL for the non-integrated case.

 app/commands.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/app/commands.c b/app/commands.c
index 7eded3c..b028f9c 100644
--- a/app/commands.c
+++ b/app/commands.c
@@ -630,7 +630,8 @@ help_manual_callback (GtkAction *action)
   GdkScreen *screen;
   DDisplay *ddisp;
   ddisp = ddisplay_active();
-  screen = gtk_widget_get_screen (GTK_WIDGET(ddisp->menu_bar));
+  screen = ddisp ? gtk_widget_get_screen (GTK_WIDGET(ddisp->shell))
+         : gdk_screen_get_default ();
   if (gtk_show_uri(screen, "ghelp:dia", gtk_get_current_event_time (), NULL)) {
     return;
   }