Object reference not set to an instance of an object.

Change the GetSelectedRows() as below -

Private Sub GetSelectedRows()
        Dim dt As DataTable
        If Session("ItemTable") Is Nothing Then
            dt = CreateTable()
            Dim dr1 As DataRow = dt.NewRow()
            dr1(0) = txtItemCode.Text
            dr1(1) = txtItemName.Text
            dr1(2) = txtStock.Text
            dr1(3) = txtPrice.Text
            dr1(4) = lblPrice.Text
            dt.Rows.Add(dr1)
        Else
            dt = CType(Session("ItemTable"),DataTable) 
            Dim dr As DataRow = dt.NewRow()'I am getting the Error hear only
            dr(0) = txtItemCode.Text
            dr(1) = txtItemName.Text
            dr(2) = txtStock.Text
            dr(3) = txtPrice.Text
            dr(4) = lblPrice.Text
            dt.Rows.Add(dr)
        End If
        Session("ItemTable") = dt
    End Sub

in getselectedrows function
first line is
Dim dt As DataTable
here you miss the new keyword
so final line will be like this
Dim dt As new DataTable

Protected Sub btnadd_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnadd.Click
        GetSelectedRows()
                                                   }"
        Me.ClientScript.RegisterStartupScript(Me.[GetType](), "UpdateParentWindow", "opener.location.reload(); window.close();", True)
 

    End Sub
 Private Sub GetSelectedRows()
        Dim dt As new DataTable ' here you need to put a new keyword 
        If Session("ItemTable") Is Nothing Then
            dt = CreateTable()
            Dim dr1 As DataRow = dt.NewRow()
            dr1(0) = txtItemCode.Text
            dr1(1) = txtItemName.Text
            dr1(2) = txtStock.Text
            dr1(3) = txtPrice.Text
            dr1(4) = lblPrice.Text
            dt.Rows.Add(dr1)
        Else
            dt = Session("ItemTable") 
            Dim dr As DataRow = dt.NewRow()'I am getting the Error hear only
            dr(0) = txtItemCode.Text
            dr(1) = txtItemName.Text
            dr(2) = txtStock.Text
            dr(3) = txtPrice.Text
            dr(4) = lblPrice.Text
            dt.Rows.Add(dr)
        End If
        Session("ItemTable") = dt
    End Sub
ublic Function CreateTable() As DataTable
        dim dt = New DataTable()
        dt.Columns.Add("ItemCode")
        dt.Columns.Add("ItemName")
        dt.Columns.Add("InStock")
        dt.Columns.Add("Price")
        dt.Columns.Add("TotalPrice")
        dt.AcceptChanges()
        Return dt
    End Function

also in CreateTable function
first line just put a dim before dt.
so hope everything gets ok