[longomatch] Fix exception in the players sorting function

[Andoni Morales Alastruey <amorales src gnome org>]

commit e42f87360c27dd081b4f7a256bf80a3a5033ee14
Author: Andoni Morales Alastruey <ylatuya gmail com>
Date:   Mon Dec 1 10:43:23 2014 +0100

    Fix exception in the players sorting function

 LongoMatch.GUI/Gui/TreeView/PlayersTreeView.cs |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/LongoMatch.GUI/Gui/TreeView/PlayersTreeView.cs b/LongoMatch.GUI/Gui/TreeView/PlayersTreeView.cs
index dbc496e..37ce3d7 100644
--- a/LongoMatch.GUI/Gui/TreeView/PlayersTreeView.cs
+++ b/LongoMatch.GUI/Gui/TreeView/PlayersTreeView.cs
@@ -50,6 +50,14 @@ namespace LongoMatch.Gui.Component
                        oa = model.GetValue(a, 0);
                        ob = model.GetValue(b, 0);
 
+                       if (oa == null && ob == null) {
+                               return 0;
+                       } else if (oa == null) {
+                               return -1;
+                       } else if (ob == null) {
+                               return 1;
+                       }
+
                        if(oa is Player)
                                return (oa as Player).Number.CompareTo((ob as Player).Number);
                        else