Hello!

Steps to reproduce the problem:
1) Create a new tag and a photo.
2) Filter to that tag.
3) Enter edit mode.
4) Remove that photo.
5) Update the filter (using the "refresh" button on the upper right corner)
6) KABOOM!

Exception thrown:
Exception in Gtk# callback delegate
  Note: Applications can use GLib.ExceptionManager.UnhandledException to handle the exception.
System.ArgumentNullException: Argument cannot be null.
Parameter name: key
  at System.Collections.Generic.Dictionary`2[System.Object,System.Object].set_Item (System.Object key, System.Object value) [0x00000] 
  at FSpot.Utils.DisposableCache`2[System.String,Gdk.Pixbuf].Add (System.String key, Gdk.Pixbuf value) [0x00000] 
  at FSpot.Widgets.Filmstrip.GetPixbuf (Int32 i, Boolean highlighted) [0x00000] 
  at FSpot.Widgets.Filmstrip.GetPixbuf (Int32 i) [0x00000] 
  at FSpot.Widgets.Filmstrip.OnExposeEvent (Gdk.EventExpose evnt) [0x00000] 
  at Gtk.Widget.exposeevent_cb (IntPtr widget, IntPtr evnt) [0x00000] 
   at GLib.ExceptionManager.RaiseUnhandledException(System.Exception e, Boolean is_terminal)
   at Gtk.Widget.exposeevent_cb(IntPtr widget, IntPtr evnt)
   at Gtk.Application.gtk_main()
   at Gtk.Application.Run()
   at Gnome.Program.Run()
   at FSpot.Driver.Main(System.String[] args)

Thanks!
Felipe.