At this point not much else is known other than the stack trace. The stack trace suggests that this happens on selection done by dragging the mouse and perhaps the document is with incorrect structure.
Exception: [ArgumentNull_Generic]
Arguments:
Debugging resource strings are unavailable. Often the key and arguments provide sufficient information to diagnose the problem. See http://go.microsoft.com/fwlink/?linkid=106663&Version=5.1.50907.00&File=mscorlib.dll&Key=ArgumentNull_Generic
Parameter name: inline
at Telerik.Windows.Documents.Model.RadDocument.GetContainingAnnotationRanges[TRangeStart](Inline inline, Predicate`1 filter, Boolean inclusive)
at Telerik.Windows.Documents.DocumentPosition.get_IsPositionInSkipPositionRange()
at Telerik.Windows.Documents.DocumentPosition.get_IsValidPosition()
at Telerik.Windows.Documents.DocumentPosition.Validator.EnsureValidPosition(DocumentPosition position, Func`1 shouldSearchForward)
at Telerik.Windows.Documents.Selection.MouseSelectionHandler.UpdateSelectionAndCaretPosition()
at Telerik.Windows.Documents.Selection.MouseSelectionHandler.RegisterDocumentMouseMove(Point position, SourceType source)
at Telerik.Windows.Documents.UI.DocumentPresenterBase.HandleMouseMoveOnPosition(Point position, SourceType source)
at Telerik.Windows.Documents.UI.DocumentPresenterBase.Owner_MouseMove(Object sender, MouseEventArgs e)
at MS.Internal.CoreInvokeHandler.InvokeEventHandler(UInt32 typeIndex, Delegate handlerDelegate, Object sender, Object args)
at MS.Internal.JoltHelper.FireEvent(IntPtr unmanagedObj, IntPtr unmanagedObjArgs, Int32 argsTypeIndex, Int32 actualArgsTypeIndex, String eventName, UInt32 flags)