[latexila] Structure: avoid a critical warning

[SÃbastien Wilmet <swilmet src gnome org>]

commit d504ecc5be20b82b95e28bcad7cc2954a26e76c2
Author: SÃbastien Wilmet <swilmet src gnome org>
Date:   Tue Jul 26 02:07:45 2011 +0200

    Structure: avoid a critical warning

 src/structure_model.vala |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/structure_model.vala b/src/structure_model.vala
index 6d17bac..b64dc62 100644
--- a/src/structure_model.vala
+++ b/src/structure_model.vala
@@ -430,7 +430,7 @@ public class StructureModel : TreeModel, GLib.Object
         regenerate_simple_lists ();
     }
 
-    public void modify_data (TreePath path, string text, TextMark end_mark)
+    public void modify_data (TreePath path, string? text, TextMark end_mark)
     {
         TreeIter iter;
         bool iter_is_valid = get_iter (out iter, path);
@@ -440,7 +440,7 @@ public class StructureModel : TreeModel, GLib.Object
 
         // modify data
         new_stamp ();
-        node.data.text = text;
+        node.data.text = text ?? "";
         node.data.end_mark = end_mark;
 
         if (emit_signals)