I have tested your code and no compile-time errors occur. Please, try to locate the error, or send a sample project, that we can analyze and fix.
Additionally, I suggest handling the RadChart.Click event instead of the RadChartItem.ActiveRegion.Click event. The event arguments provide all the necessary data: Series, SeriesItem, etc.
Note, that this event is fired in some other cases, so you need to check if SeriesItem and/or Series are not null.