Checkboxes in DataGrid

I have a GridView containing a column with checkboxes. The header template has a a checkbox to select all the other checkboxes in the column.

When I tick and do an event, I am getting this error:

Object reference not set to an instance of an object. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

on this line:

ifchecked = DirectCast(item.FindControl("ckTransferItem"), CheckBox).Checked

GridView Code:

<asp:DataGrid id="Dgcabinet" runat="server" CssClass="datagrids psLink" Width="100%" BorderStyle="None"
ForeColor="Transparent" GridLines="Horizontal" PagerStyle-Mode="NumericPages" AllowPaging="True"
AllowSorting="True" AutoGenerateColumns="False" PageSize="200">
    <ItemStyle CssClass="datagrids psLink"></ItemStyle>
    <HeaderStyle ForeColor="White" CssClass="datagridsHeader"></HeaderStyle>
    <Columns>
    <asp:BoundColumn Visible="False" DataField="file_pk"></asp:BoundColumn>
    <asp:BoundColumn DataField="file_pk" SortExpression="file_pk" HeaderText="&amp;nbsp;File Number">
    <HeaderStyle HorizontalAlign="Center"></HeaderStyle>
    <ItemStyle HorizontalAlign="Center"></ItemStyle>
    </asp:BoundColumn>
    <asp:BoundColumn Visible="False" DataField="file_subject"></asp:BoundColumn>
    <asp:BoundColumn DataField="file_subject" SortExpression="file_subject" HeaderText="&amp;nbsp;File Name">
    <HeaderStyle HorizontalAlign="Center"></HeaderStyle>
    <ItemStyle HorizontalAlign="Center"></ItemStyle>
    </asp:BoundColumn>
    <asp:BoundColumn Visible="False" DataField="cab_pk"></asp:BoundColumn>
    <asp:BoundColumn DataField="cab_pk" SortExpression="cab_pk" HeaderText="&amp;nbsp;Storage Number">
    <HeaderStyle HorizontalAlign="Center"></HeaderStyle>
    <ItemStyle HorizontalAlign="Center"></ItemStyle>
    </asp:BoundColumn>
    <asp:BoundColumn Visible="False" DataField="comp_part"></asp:BoundColumn>
    <asp:BoundColumn DataField="comp_part" SortExpression="comp_part" HeaderText="&amp;nbsp;Compartment No.">
    <HeaderStyle HorizontalAlign="Center"></HeaderStyle>
    <ItemStyle HorizontalAlign="Center"></ItemStyle>
    </asp:BoundColumn>
    <asp:BoundColumn Visible="False" DataField="dept_pk"></asp:BoundColumn>
    <asp:BoundColumn DataField="dept_code" SortExpression="dept_code" HeaderText="&amp;nbsp;Department Code">
    <HeaderStyle HorizontalAlign="Center"></HeaderStyle>
    <ItemStyle HorizontalAlign="Center"></ItemStyle>
    </asp:BoundColumn>
    <asp:BoundColumn DataField="locationno" SortExpression="locationno" HeaderText="&amp;nbsp;Storage Location">
    <HeaderStyle HorizontalAlign="Center"></HeaderStyle>
    <ItemStyle HorizontalAlign="Center"></ItemStyle>
    </asp:BoundColumn>
    <asp:TemplateColumn HeaderText="Tick to Transfer">
    <HeaderStyle HorizontalAlign="Center"></HeaderStyle>
    <ItemStyle HorizontalAlign="Center"></ItemStyle>
    <ItemTemplate>
    <asp:CheckBox ID="ckTransferItem" Runat="server" AutoPostBack="false"></asp:CheckBox>
    </ItemTemplate>
    </asp:TemplateColumn>
    </Columns>
    <PagerStyle CssClass="datagridsFooter psLink" Mode="NumericPages"></PagerStyle>
</asp:DataGrid>

Button Event Handler:

Private Sub btnbulktransfer_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnbulktransfer.Click
        'get number of rows in datagrid
        Dim rowcount As Integer = Dgcabinet.Items.Count
        Dim itemrow As Integer = 0
        Dim item As DataGridItem
        Dim ifchecked As Boolean = False
        Dim getcompartment As Integer
        Dim getnewstate As Integer
        Dim getlocation As Integer
        Dim statetocomment As String
        Dim lbresult As Boolean = False
        Dim liUserRoleId As Integer
        Dim liUserId As Integer
        Dim switch As Boolean = False
        Dim linewdepartment As Integer
        Dim licurrentdepartment As Integer
        Dim lioldstates As Integer
        Dim listateid As Integer
        Dim lsImriehelRepository As String
        lioldstates = ddloldstates.SelectedValue


        licurrentdepartment = ddldepts.SelectedValue

        liUserRoleId = Convert.ToInt64(lcPrimitives.GetSessionCache("_USERROLE_ID"))
        liUserId = Convert.ToInt64(lcPrimitives.GetSessionCache("_USER_ID"))


        getlocation = ddllocation.SelectedValue
        getcompartment = ddlstorage.SelectedValue
        getnewstate = ddlnewstates.SelectedValue
        statetocomment = ddlnewstates.Items(ddlnewstates.SelectedIndex).Text
        linewdepartment = BLeRecords.getFileDepartment(getcompartment)
        listateid = ddldepts.SelectedValue

        'check if storage is in the imriehel repository
        lsImriehelRepository = [Global]._eRecordsPortalConstants.eRecordsCONST_ToImriehelRepository
        If linewdepartment = lsImriehelRepository Then
            linewdepartment = licurrentdepartment
        End If

        'check that all 'transfer to' dropdown options are selected
        If (getlocation = 0) Then
            lblError.Visible = True
            lblError.Text = "!Error. Please check location dropdown."
        ElseIf (getcompartment = 0) Then
            lblError.Visible = True
            lblError.Text = "!Error. Please check Storage dropdown."
        ElseIf (getnewstate = 0) Then
            lblError.Visible = True
            lblError.Text = "!Error. Please check Status dropdown."
        Else
            Dim fileIDs As ArrayList = New ArrayList
            Dim department As Integer
            ClearMessages()
            For itemrow = 0 To rowcount - 1
                ' read first row to check if the tickbox is checked
                item = Dgcabinet.Items(itemrow)
                ifchecked = DirectCast(item.FindControl("ckTransferItem"), CheckBox).Checked
                department = Integer.Parse(item.Cells(8).Text())

                If ifchecked Then
                    Dim filenumber As String = (item.Cells(0).Text())
                    fileIDs.Add(filenumber)
                    switch = True
                End If
            Next


            'check if tickbox are ticked. If not returns error.
            If switch = False Then
                lblError.Visible = True
                lblError.Text = "!Error. Please check tick box"
            Else
                lbresult = BLeRecords.updateBulkTransferBatch(fileIDs, getcompartment, linewdepartment, getnewstate, statetocomment, liUserId, listateid)

                If lbresult = True Then
                    lblmessage.Text = "File transfer succesful"
                    lblmessage.Visible = True
                    DisplaySortedData("", lioldstates, licurrentdepartment, False)
                Else
                    lblError.Visible = True
                    lblError.Text = "!Error. File transfer Failed. Contact Administrator"
                End If
            End If

        End If
    End Sub

Can anyone explain why?

If the checkbox is checked it should be set to True no?