How to reproduce: 
Me.RadPivotGrid1.DataSource = Nothing

Workaround: 
DirectCast(Me.RadPivotGrid1.DataProvider, LocalDataSourceProvider).ItemsSource = Nothing
Dim viewModel = Me.RadPivotGrid1.PivotGridElement.GetType().GetField("viewModel", BindingFlags.Instance Or BindingFlags.NonPublic).GetValue(Me.RadPivotGrid1.PivotGridElement)
viewModel.GetType().GetProperty("DataProvider").SetValue(viewModel, Nothing)