Object reference not set to an instance of an object in report view using vb

hi
I need to disable/hide the excel in reportviewer. But error persist "Object reference not set to an instance of an object. "
I debugged, there shows line is " Dim ListRenderingExtensions As System.Reflection.MethodInfo = m_previewService.FieldType.GetMethod("ListRenderingExtensions", Flags)" 
This code taken from this forum (asp.net forum)
Pls help me.
Protected Sub DisableFormat(ByRef viewer As ReportViewer, ByVal formatName As String) Const Flags As System.Reflection.BindingFlags = System.Reflection.BindingFlags.NonPublic + System.Reflection.BindingFlags.Public + System.Reflection.BindingFlags.Instance Dim m_previewService As System.Reflection.FieldInfo = viewer.LocalReport.GetType().GetField("m_previewService", Flags) Dim ListRenderingExtensions As System.Reflection.MethodInfo = m_previewService. ...