<ext:ComboBox ID="cbWarehouseID" runat="server" FieldLabel="Warehouse" LabelWidth="20"
                                                        ValueField="WarehouseID" DisplayField="Name">
                                                        <Store>
                                                            <ext:Store ID="dsWarehouses" runat="server" AutoLoad="false">
                                                                <Proxy>
                                                                    <ext:HttpProxy Url='/BaseInfo/GetWarehouseList/' />
                                                                </Proxy>
                                                                <Reader>
                                                                    <ext:JsonReader IDProperty="WarehouseID" Root="data" TotalProperty="total">
                                                                        <Fields>
                                                                            <ext:RecordField Name="WarehouseID" />
                                                                            <ext:RecordField Name="Name" />
                                                                            <ext:RecordField Name="Description" />
                                                                        </Fields>
                                                                    </ext:JsonReader>
                                                                </Reader>
                                                                <BaseParams>
                                                                    <ext:Parameter Name="limit" Value="30" Mode="Raw" />
                                                                    <ext:Parameter Name="start" Value="0" Mode="Raw" />
                                                                    <ext:Parameter Name="dir" Value="DESC" />
                                                                    <ext:Parameter Name="sort" Value="WarehouseID" />
                                                                </BaseParams>
                                                                <SortInfo Field="WarehouseID" Direction="DESC" />
                                                            </ext:Store>
                                                        </Store>
                                                        <Listeners>
                                                            <AfterRender Handler="dsWarehouses.load();if (cbWarehouseID.store.getAt(0)){var firstValue =cbWarehouseID.store.getAt(0).get('Name');cbWarehouseID.setValue(firstValue)};" />
                                                        </Listeners>
                                                    </ext:ComboBox>
it does not work?

please help me,thanks!