This is expected behavior, because when the user tries to delete an appointment, a confirmation dialog is shown first and after the deletion is confirmed, the events are fired. So when you cancel ShowDialog event, you prevent this dialog from displaying. In order to workaround this, you can set
DefaultDialogResult property of the ShowDialogEventArgs to true like this:
sender, ShowDialogEventArgs e)
All the best,
the Telerik team
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