why can not Trigger the event       when sender is "ClearCanvas.Desktop.Workspace"

  obj.InternalActiveChanged += delegate(object sender, EventArgs e)
            {
                OnItemActivationChangedInternal(new ItemEventArgs((T)sender));
            };

can not exec the event fun:

        protected sealed override void OnItemActivationChangedInternal(ItemEventArgs args)
        {
            if (args.Item.Active)
            {
                // activated
                Workspace lastActive = _activeWorkspace;
                // set this prior to firing any events, so that a call to ActiveWorkspace property will return correct value
                _activeWorkspace = args.Item;
                if (lastActive != null)
                {
                    lastActive.RaiseActiveChanged();
                }
                _activeWorkspace.RaiseActiveChanged();
               
            }
        }

so activeWorkspace is null throw the execption.

please tell me?