RSS Reader Error - NullReferenceException

Imports System.Xml
Imports System.Xml.XPath

Public Class FormRSS

    Private mRssUrl As String

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        mRssUrl = String.Empty

    End Sub

    Private Sub tsRssGo_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles tsRssGo.Click

                'Set RSS Feed URL
                mRssUrl = "http://news.google.co.kr/news?pz=1&cf=all&ned=kr&hl=ko&output=rss"
                'ListView Clear
                tvwRss.Nodes.Clear()
                'Create New XMLDocument Variable
                Dim doc As New XmlDocument()
                Try
                    'Load RSS from RSS URL
                    doc.Load(mRssUrl)
                Catch ex1 As Exception
                    'Showin Error Msg
                    MessageBox.Show(ex1.Message)
                    Return
                End Try
                Dim navigator As XPathNavigator = doc.CreateNavigator()
                Try
                    'Get Post Title form RSS
                    Dim nodes As XPathNodeIterator = navigator.Select("/rss/channel/item/title")
                    While nodes.MoveNext
                        Dim node As XPathNavigator = nodes.Current
                        Dim tmp As String = node.Value.Trim()
                        tmp = tmp.Replace(ControlChars.CrLf, "")
                        tmp = tmp.Replace(ControlChars.Lf, "")
                        tmp = tmp.Replace(ControlChars.Cr, "")
                        tmp = tmp.Replace(ControlChars.FormFeed, "")
                        tmp = tmp.Replace(ControlChars.NewLine, "")
                        'Post Title add to ListView
                        tvwRss.Nodes.Add(tmp)
                    End While
                    Dim position As Integer = 0
                    ' Get the links from the RSS feed
                    Dim nodesLink As XPathNodeIterator = navigator.Select("/rss/channel/item/link")
                    While nodesLink.MoveNext
                        Dim node As XPathNavigator = nodesLink.Current
                        Dim tmp As String = node.Value.Trim()
                        tmp = tmp.Replace(ControlChars.CrLf, "")
                        tmp = tmp.Replace(ControlChars.Lf, "")
                        tmp = tmp.Replace(ControlChars.Cr, "")
                        tmp = tmp.Replace(ControlChars.FormFeed, "")
                        tmp = tmp.Replace(ControlChars.NewLine, "")
                        ' Child Node Add to Parent Nodes
                        tvwRss.Nodes(position).Nodes.Add(tmp)
                        PostBody(position) = tmp   '<----   System.NullReferenceException   Error!!!
                        position += 1
                    End While
                Catch ex As Exception
                    MessageBox.Show(ex.Message, "Load Fail")
                End Try
            Catch ex2 As Exception
                MessageBox.Show(ex2.ToString(), "Initialization Fail")
            End Try
        End If
    End Sub