[gimp] app: gimp_dockable_set_dockbook() must be able to take NULL

[Martin Nordholts <martinn src gnome org>]

commit 1e62e58f61f56962c99c2f770ac56e638f1e437a
Author: Martin Nordholts <martinn src gnome org>
Date:   Tue Jan 5 10:28:16 2010 +0100

    app: gimp_dockable_set_dockbook() must be able to take NULL

 app/widgets/gimpdockable.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/app/widgets/gimpdockable.c b/app/widgets/gimpdockable.c
index 3eac352..a955c9d 100644
--- a/app/widgets/gimpdockable.c
+++ b/app/widgets/gimpdockable.c
@@ -925,7 +925,8 @@ gimp_dockable_set_dockbook (GimpDockable *dockable,
                             GimpDockbook *dockbook)
 {
   g_return_if_fail (GIMP_IS_DOCKABLE (dockable));
-  g_return_if_fail (GIMP_IS_DOCKBOOK (dockbook));
+  g_return_if_fail (dockbook == NULL ||
+                    GIMP_IS_DOCKBOOK (dockbook));
 
   dockable->p->dockbook = dockbook;
 }