下拉编辑列表框多选Devexpress

Question

这是我的代码

<dx:GridViewDataTextColumn Width="10%" FieldName="TestAlert" CellStyle-HorizontalAlign="Center">
    <DataItemTemplate>
        <dx:ASPxDropDownEdit ClientInstanceName="checkComboBox" ID="ASPxDropDownEdit1" runat="server" AnimationType="None" >
             <DropDownWindowStyle BackColor="#EDEDED" />
            <DropDownWindowTemplate>

                <dx:ASPxListBox Width="100%" ID="listBox" ClientInstanceName="checkListBox" SelectionMode="CheckColumn"
                        runat="server">
                        <Border BorderStyle="None" />
                        <BorderBottom BorderStyle="Solid" BorderWidth="1px" BorderColor="#DCDCDC" />
                        <Items>

                            <dx:ListEditItem Text="(Select all)" />
                            <dx:ListEditItem Text="Alert" Value="1" />
                            <dx:ListEditItem Text="Delete" Value="2" />

                        </Items>
                        <ClientSideEvents SelectedIndexChanged="OnListBoxSelectionChanged" />
                    </dx:ASPxListBox>
                    <table style="width: 100%">
                        <tr>
                            <td style="padding: 4px">
                                <dx:ASPxButton ID="ASPxButton1" AutoPostBack="False" runat="server" Text="Close" style="float: right">
                                    <ClientSideEvents Click="function(s, e){ checkComboBox.HideDropDown(); }" />
                                </dx:ASPxButton>
                            </td>
                        </tr>
                    </table>
            </DropDownWindowTemplate>
            <%-- <ClientSideEvents TextChanged="SynchronizeListBoxValues" DropDown="SynchronizeListBoxValues" />--%>

        </dx:ASPxDropDownEdit>

任何人都可以帮我选择ex：ALert的值，同时点击按钮......

ListBox listBox = (ListBox)LawyerASPxGridView.FindRowCellTemplateControl(e.VisibleIndex, LawyerASPxGridView.DataColumns["ListBox"], "listBox");
listBox.Items.FindByText("Alert").Selected = true;

但是这会抛空......请帮帮我