When the ItemsSource (ObservableCollection) is empty (Count = 0) calling BeginInsert does cause the AddingNewDataItem event to fire. This works fine when ItemsSource has at least 1 item.
I have tried to reproduce the issue you reported, but unfortunately without any success. Could you take a look at the sample attached and let me know whether you can get the same behavior on it as well ?
Best wishes,
Maya
the Telerik team
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