Error in exporting selected rows from Gridview to Excel - asp.net

Question

I am trying to export selected columns from GridView to Excel, and every time I run the code an error pops up with some new .dll file name. What could be the issue? Please find the code below for your reference.
<asp:Panel ID="general" runat="server" BorderColor="#333333" BorderStyle="Double" CssClass="emailpnl" Width="800px">
<asp:GridView ID="bestpractgrv" runat="server" AutoGenerateColumns="False" OnLoad="bestpractgrv_Load" CellPadding="3" GridLines="Vertical" Width="100%" BackColor="White" BorderColor="#999999" BorderStyle="None" BorderWidth="1px" AllowPaging="True" OnPageIndexChanging = "OnPaging">
<AlternatingRowStyle BackColor="Gainsboro" />
<Columns>
<asp:TemplateField>
<HeaderTemplate>
<asp:CheckBox ID="chkboxSelectAll" runat="server" onclick="CheckAllEmp(this);" />
</HeaderTemplate>
<ItemTemplate>
<asp:CheckBox ID ="boolcb" runat="server" />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="ID">
<ItemTemplate>
<asp:Label ID="idlbl" runat="server" Text='<%# Bind("iID")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Discipline ID">
<ItemTemplate>
<asp:Label ID="discplnlbl" runat="server" Text='<%# Bind("iDisciplineID")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Date Created">
<ItemTemplate>
<asp:Label ID="dateclbl" runat="server" Text='<%# Bind("dDateCreated")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Title">
<ItemTemplate>
<asp:Label ID="bptitlelbl" runat="server" Text='<%# Bind("cBPTitle")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Columns>
<FooterStyle BackColor="#CCCCCC" ForeColor="Black" />
<HeaderStyle BackColor="#000084" Font-Bold="True" ForeColor="White" />
<PagerStyle BackColor="#999999" ForeColor="Black" HorizontalAlign="Center" />
<RowStyle BackColor="#EEEEEE" ForeColor="Black" HorizontalAlign="Center" />
<SelectedRowStyle BackColor="#008A8C" Font-Bold="True" ForeColor="White" />
<SortedAscendingCellStyle BackColor="#F1F1F1" />
<SortedAscendingHeaderStyle BackColor="#0000A9" />
<SortedDescendingCellStyle BackColor="#CAC9C9" />
<SortedDescendingHeaderStyle BackColor="#000065" />
</asp:GridView>
</asp:Panel>
VB Code
Private Sub PopulateCheckBoxArray()
Dim CheckBoxArray As ArrayList
If ViewState("CheckBoxArray") IsNot Nothing Then
CheckBoxArray = DirectCast(ViewState("CheckBoxArray"), ArrayList)
Else
CheckBoxArray = New ArrayList()
End If
Dim CheckBoxIndex As Integer
Dim CheckAllWasChecked As Boolean = False
Dim chkAll As CheckBox = DirectCast(bestpractgrv.HeaderRow.Cells(0).FindControl("chkboxSelectAll"), CheckBox)
Dim checkAllIndex As String = "chkboxSelectAll-" & bestpractgrv.PageIndex
If chkAll.Checked Then
If CheckBoxArray.IndexOf(checkAllIndex) = -1 Then
CheckBoxArray.Add(checkAllIndex)
End If
Else
If CheckBoxArray.IndexOf(checkAllIndex) <> -1 Then
CheckBoxArray.Remove(checkAllIndex)
CheckAllWasChecked = True
End If
End If
For i As Integer = 0 To bestpractgrv.Rows.Count - 1
If bestpractgrv.Rows(i).RowType = DataControlRowType.DataRow Then
Dim chk As CheckBox = DirectCast(bestpractgrv.Rows(i).Cells(0).FindControl("boolcb"), CheckBox)
CheckBoxIndex = bestpractgrv.PageSize * bestpractgrv.PageIndex + (i + 1)
If chk.Checked Then
If CheckBoxArray.IndexOf(CheckBoxIndex) = -1 AndAlso Not CheckAllWasChecked Then
CheckBoxArray.Add(CheckBoxIndex)
End If
Else
If CheckBoxArray.IndexOf(CheckBoxIndex) <> -1 OrElse CheckAllWasChecked Then
CheckBoxArray.Remove(CheckBoxIndex)
End If
End If
End If
Next
ViewState("CheckBoxArray") = CheckBoxArray
End Sub
And I get the error on following line
Dim chkAll As CheckBox = DirectCast(bestpractgrv.HeaderRow.Cells(0).FindControl("chkboxSelectAll"), CheckBox)
This is the error message
An exception of type 'System.NullReferenceException' occurred in App_Web_pqx0e3wy.dll but was not handled in user code
After every compilation the dll name changes, while App_Web_XXXX.dll remains constant