[banshee] [CoverArtEditor] Fix a NRE (bgo#608844)

[Alexander Kojevnikov <alexk src gnome org>]

commit 8671b92e2721b6baa954797c53511108a46c4e26
Author: Alexander Kojevnikov <alexander kojevnikov com>
Date:   Thu Feb 4 22:20:18 2010 +1100

    [CoverArtEditor] Fix a NRE (bgo#608844)

 .../Banshee.Collection.Gui/CoverArtEditor.cs       |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/src/Core/Banshee.ThickClient/Banshee.Collection.Gui/CoverArtEditor.cs b/src/Core/Banshee.ThickClient/Banshee.Collection.Gui/CoverArtEditor.cs
index 21f1e44..b4fb28a 100644
--- a/src/Core/Banshee.ThickClient/Banshee.Collection.Gui/CoverArtEditor.cs
+++ b/src/Core/Banshee.ThickClient/Banshee.Collection.Gui/CoverArtEditor.cs
@@ -192,7 +192,10 @@ namespace Banshee.Collection.Gui
                     if (Banshee.IO.File.Exists (uri)) {
                         Banshee.IO.File.Delete (uri);
                     }
-                    ServiceManager.Get<ArtworkManager> ().ClearCacheFor (track.ArtworkId);
+                    var artwork_id = track.ArtworkId;
+                    if (artwork_id != null) {
+                        ServiceManager.Get<ArtworkManager> ().ClearCacheFor (track.ArtworkId);
+                    }
                 }
             }