To reproduce:
-  Use the cut option in the context menu then  dispose the rich text editor and collect the garbage.

Workaround:
Telerik.WinControls.RichTextEditor.UI.ContextMenu menu = textEditor.RichTextBoxElement.ContextMenu as Telerik.WinControls.RichTextEditor.UI.ContextMenu;
textEditor.RichTextBoxElement.ContextMenu = null;

RadDropDownMenu ddm = menu.GetType().GetField("radDropDownMenu", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(menu) as RadDropDownMenu;