RadComboBox Cell text in RadGridGriew

Question

Am Recording web Application. That is silver light application.I have a RadGridView with a column that is RadComboBox:

when am selecting item from RadComboBox item is not selecting while running.that step is failing.

Find the below code :

SilverlightApp app = ActiveBrowser.SilverlightApps()[0];
RadGridView grid = app.Find.ByAutomationId<RadGridView>("RadGridViewName");
Assert.IsNotNull(grid);
GridViewRow row = grid.Rows[2];
row.User.Click(MouseClickType.LeftDoubleClick);

Telerik.WebAii.Controls.Xaml.RadComboBox   
cb=row.Find.ByType<Telerik.WebAii.Controls.Xaml.RadComboBox>();

cb.SelectItem("Football", true);
System.Threading.Thread.Sleep(1000);

But am getting out of index error. Let me know where am doing mistake....