I tried to delegate an action and got an unexpected error...

This was shown:

System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
   bei Wieldy.Client.Modes.Todo.Filters.ReviewDelegatedFilter.Assign(IItemBase ?)
   bei Wieldy.Client.Modes.Todo.ItemRibbon.AssignItemClick(Object sender, RoutedEventArgs e)
   bei System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   bei System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   bei System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
   bei System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
   bei System.Windows.Controls.MenuItem.InvokeClickAfterRender(Object arg)
   bei System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
   bei System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)