Steps to reproduce:
1. Enable Color Management
2. View -> Tag Icons -> Hidden


Stack trace:
*** Running uninstalled f-spot ***
** Running f-spot in Debug Mode **
** Running Mono with --debug   **
[Info  21:07:25.171] Initializing DBus
[Debug 21:07:25.367] DBusInitialization took 0.181543s
[Info  21:07:25.367] Initializing Mono.Addins
[Debug 21:07:26.285] Mono.Addins Initialization took 0.918285s
[Info  21:07:26.296] Starting new FSpot server (f-spot 0.5.0.3)
[Debug 21:07:26.636] Db Initialization took 0.166925s
[Debug 21:07:27.605] QueryToTemp took 0.101347s : SELECT id, time, uri, description, roll_id, default_version_id, rating, md5_sum FROM photos  WHERE id NOT IN (SELECT photo_id FROM photo_tags WHERE tag_id = 2) ORDER BY  time DESC
[Debug 21:07:27.875] Query took 0.016969s : SELECT * FROM photoquery_temp_0 LIMIT 100 OFFSET 0
[Debug 21:07:27.890] PhotosPerMonth took 0.006281s
[Debug 21:07:27.893] TimeAdaptor REAL Reload took 0.215917s
[Debug 21:07:27.983] QueryToTemp took 0.001865s : SELECT id, time, uri, description, roll_id, default_version_id, rating, md5_sum FROM photos  WHERE id NOT IN (SELECT photo_id FROM photo_tags WHERE tag_id = 2) ORDER BY  time ASC
[Debug 21:07:27.991] Query took 0.001039s : SELECT * FROM photoquery_temp_0 LIMIT 100 OFFSET 0
[Debug 21:07:28.025] open uri = file:///home/wasja/Photos/2004/12/07/18593-1.png
[Debug 21:07:28.049] open uri = file:///home/wasja/Photos/2004/12/07/18593-1.png
[Debug 21:07:28.109] Reloading the query took 0.128114s
[Debug 21:07:28.194] PhotosPerMonth took 0.004675s
[Debug 21:07:28.194] TimeAdaptor REAL Reload took 0.208148s
[Info  21:07:28.225] Hack for gnome-settings-daemon engaged

(f-spot:30873): GdkPixbuf-WARNING **: GdkPixbufLoader finalized without calling gdk_pixbuf_loader_close() - this is not allowed. You must explicitly end the data stream to the loader before dropping the last reference.
[Debug 21:07:28.403] LookupItem took 0.000432s
[Debug 21:07:28.403] IndexOf took 0.000253s : SELECT ROWID AS row_id FROM photoquery_temp_0 WHERE id = 1
Exception in Gtk# callback delegate
  Note: Applications can use GLib.ExceptionManager.UnhandledException to handle the exception.
System.NullReferenceException: Object reference not set to an instance of an object
  at TagSelectionWidget.IconDataFunc (Gtk.TreeViewColumn column, Gtk.CellRenderer renderer, TreeModel model, TreeIter iter) [0x0004e] in /home/wasja/development/f-spot/trunk/src/TagSelectionWidget.cs:197 
  at GtkSharp.TreeCellDataFuncWrapper.NativeCallback (IntPtr tree_column, IntPtr cell, IntPtr tree_model, IntPtr iter, IntPtr data) [0x00000] 
   at GLib.ExceptionManager.RaiseUnhandledException(System.Exception e, Boolean is_terminal)
   at GtkSharp.TreeCellDataFuncWrapper.NativeCallback(IntPtr tree_column, IntPtr cell, IntPtr tree_model, IntPtr iter, IntPtr data)
   at GtkSharp.TreeCellDataFuncWrapper.NativeCallback(IntPtr , IntPtr , IntPtr , IntPtr , IntPtr )
   at Gtk.Application.gtk_main()
   at Gtk.Application.gtk_main()
   at Gtk.Application.Run()
   at Gnome.Program.Run()
   at FSpot.Driver.Main(System.String[] args) in /home/wasja/development/f-spot/trunk/src/main.cs:line 339
[Debug 21:07:49.159] Finalizer called on FSpot.Widgets.Filmstrip. Should be Disposed
[Debug 21:07:49.160] Finalizer called on FSpot.Tag. Should be Disposed
[Debug 21:07:49.161] Finalizer called on Banshee.Database.QueuedSqliteDatabase. Should be Disposed
[Debug 21:07:49.161] Finalizer called on Db. Should be Disposed
[Debug 21:07:49.163] Finalizer called on FSpot.PhotoView. Should be Disposed
[Debug 21:07:49.164] Finalizer called on FSpot.ThumbnailCache. Should be Disposed
[Debug 21:07:49.164] Finalizer called on FSpot.Utils.DisposableCache`2[System.String,Gdk.Pixbuf]. Should be Disposed
[Debug 21:07:49.171] Finalizer called on FSpot.Category. Should be Disposed
[Debug 21:07:49.171] Finalizer called on FSpot.Category. Should be Disposed
[Debug 21:07:49.171] Finalizer called on FSpot.Category. Should be Disposed
[Debug 21:07:49.171] Finalizer called on FSpot.Category. Should be Disposed
[Debug 21:07:49.171] Finalizer called on FSpot.Category. Should be Disposed
[Debug 21:07:49.178] Finalizer called on FSpot.Utils.DisposableCache`2[System.String,Gdk.Pixbuf]. Should be Disposed


Other information:
ps allready fixed
(TagSelectionWidget.cs, line 194: change to "if (tag.SizedIcon != null) {" )