Object Reference Not Set To An Instance Of An Object (When I Am Clicking On The Delete Button But Still Rows Are Deleting)

<asp:gridview id="PurchageOrderGrid" runat="server" autogeneratecolumns="false" align="center">
    <columns>
        <asp:BoundField DataField="RowNumber" HeaderText="#" />
        <asp:TemplateField HeaderText="Item ID رقم الصنف">
            <itemtemplate>
                <asp:TextBox ID="ItemID" runat="server">
            </itemtemplate>
        </asp:TemplateField>
         <asp:TemplateField HeaderText="Description الوصف">
            <itemtemplate>
                <asp:TextBox ID="Desc" runat="server">
            </itemtemplate>
        </asp:TemplateField>
         <asp:TemplateField HeaderText="QTY الكمية">
            <itemtemplate>
                <asp:TextBox ID="QTY" runat="server">
            </itemtemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Total Price إجمالي المبلغ">
            <itemtemplate>
                    <asp:TextBox ID="TotalPrice" runat="server">
            </itemtemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Type النوع">
            <itemtemplate>
                <asp:TextBox ID="Type" runat="server">
            </itemtemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Customer العميل">
            <itemtemplate>
                <asp:TextBox ID="Customer" runat="server">
            </itemtemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Comment ملاحظات">
            <itemtemplate>
                <asp:TextBox ID="Comment" runat="server">
            </itemtemplate>
        </asp:TemplateField>
        <asp:TemplateField>
            <itemtemplate>
                <asp:ImageButton ID="btnDelete" runat="server" ImageUrl="delete.jpg" title="Delete حذف" OnClick="RemoveItemsButton_Click" />
            </itemtemplate>
        </asp:TemplateField>
    </columns>
</asp:gridview>