[longomatch] Add a safety check to prevent a crash.

[Andoni Morales Alastruey <amorales src gnome org>]

commit 2131b0db5032c39bb8f0fd5913ef08114f85c611
Author: Julien Moutte <julien fluendo com>
Date:   Sat Mar 14 13:48:10 2015 +0100

    Add a safety check to prevent a crash.
    
    Looks like we can get disposed twice and Objects could be null in ClearSelection if we call it once 
disposed.

 LongoMatch.Drawing/Canvas.cs |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/LongoMatch.Drawing/Canvas.cs b/LongoMatch.Drawing/Canvas.cs
index 3b05532..affce58 100644
--- a/LongoMatch.Drawing/Canvas.cs
+++ b/LongoMatch.Drawing/Canvas.cs
@@ -62,6 +62,7 @@ namespace LongoMatch.Drawing
 
                protected virtual void Dispose (bool disposing)
                {
+                       // FIXME: Should we check if we are disposed already ?
                        if (disposing) {
                                ClearObjects ();
                                Objects = null;
@@ -271,8 +272,10 @@ namespace LongoMatch.Drawing
                                ICanvasSelectableObject po = sel.Drawable as ICanvasSelectableObject;
                                po.Selected = false;
                        }
-                       foreach (ICanvasSelectableObject cso in Objects) {
-                               cso.Selected = false;
+                       if (Objects != null) {
+                               foreach (ICanvasSelectableObject cso in Objects) {
+                                       cso.Selected = false;
+                               }
                        }
                        Selections.Clear ();
                }