If you reference both Telerik.Windows.Controls.GridView and Telerik.Windows.Controls.Data assemblies and you try to access the ControlPanelItemCollection class in XAML using the "telerik" schema, an error appears. This happens because classes with this name are available in both assembly and both classes are mapped to the "telerik" schema.

To work this around, use a concrete namespace instead of the "telerik" schema. For example:

 xmlns:gridView="clr-namespace:Telerik.Windows.Controls.GridView;assembly=Telerik.Windows.Controls.GridView"