[baobab] Chart: do not try to set the root if model is NULL

[Stefano Facchini <sfacchini src gnome org>]

commit f092863d8c18ae32ec0301a255c347809c182a7d
Author: Stefano Facchini <stefano facchini gmail com>
Date:   Mon Jul 22 21:40:52 2013 +0200

    Chart: do not try to set the root if model is NULL

 src/baobab-chart.vala |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/src/baobab-chart.vala b/src/baobab-chart.vala
index ec26102..17b7cee 100644
--- a/src/baobab-chart.vala
+++ b/src/baobab-chart.vala
@@ -136,6 +136,10 @@ namespace Baobab {
         Gtk.TreeRowReference? root_;
         public Gtk.TreePath? root {
             set {
+                if (model == null) {
+                    return;
+                }
+
                 if (root_ != null) {
                     var current_root = root_.get_path ();
                     if (current_root != null && value != null && current_root.compare (value) == 0) {