pdb: oups! I previously edited a generated file.

This is the PDB I had to edit instead of app/pdb/message-cmds.c which is
generated.

pdb/groups/message.pdb

@@ -39,7 +39,7 @@ HELP
   if (gimp->plug_in_manager->current_plug_in)
     domain = gimp_plug_in_get_undo_desc (gimp->plug_in_manager->current_plug_in);
   gimp_show_message (gimp, G_OBJECT (progress), GIMP_MESSAGE_WARNING,
-                     domain, message);
+                     domain, message, NULL);
 }
 CODE
     );