Aspx Comobox Selectedindex changed event problem

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DevExpress

Dear Sir/Madam,

In my page am using Devexpress combobox.
Selected index change event time to show selected item Null and selectedindex=-1


<dx:ASPXComboBox ID="CmbItemDesc" runat="server" Width="400px" DropDownWidth="550px" AutoPostBack="true"
EnableCallbackMode="true"
IncrementalFilteringMode="Contains" FilterMinLength="1"
IncrementalFilteringDelay="100" DropDownRows="10" CssClass="textbox_Autocomplete" DropDownStyle="DropDownList"
ValueType="System.String" CallbackPageSize="20" OnDataBound="CmbItemDesc_DataBound"
OnSelectedIndexChanged="CmbItemDesc_SelectedIndexChanged" ShowLoadingPanelImage="true" ShowShadow="true"
OnItemsRequestedByFilterCondition="CmbItemDesc_ItemsRequestedByFilterCondition"
OnItemRequestedByValue="CmbItemDesc_ItemRequestedByValue" ValueField="Item_desc">

<Columns>

<dx:ListBoxColumn FieldName="Uni_Item" visible="false"/>
<dx:ListBoxColumn FieldName="Generic_Desc" Caption="Generic" Width="210px"/>
<dx:ListBoxColumn FieldName="Item_desc" Caption="Description" Width="210px"/>
<dx:ListBoxColumn FieldName="Item_Code" Caption="Item Code" Width="60px" />
<dx:ListBoxColumn FieldName="Batch_No" Caption="Batch No" Width="70px" />
<dx:ListBoxColumn FieldName="Expiry_Date" Caption="Expiry Date" Width="70px" />
<dx:ListBoxColumn FieldName="MRP" Caption="MRP" Width="60px"/>
<dx:ListBoxColumn FieldName="Qty" Caption="Qty" Width="60px"/>
</Columns>

</dx:ASPXComboBox>