Getselectedrows of dev express gridviw method is not working

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C#

.NET

DevExpress

Hii ,

I have one gridview ,where we can select only one row at a time.
I have one button outside the gridview and want to get records of selected rows of gridview.

I have used callback control to get this records using getselectedvalues methd but not getting any values

<dx:ASPxPopupControl ID="pcPendingApprovals" runat="server" RenderMode="Lightweight" ClientInstanceName="pcPendingApprovals"
    CloseAction="CloseButton" Modal="true" ShowCloseButton="true" Width="700px"
    PopupHorizontalAlign="WindowCenter" PopupVerticalAlign="WindowCenter"
    HeaderText="Plans Awaiting Approval" PopupAnimationType="Fade" EnableViewState="False">
    <ContentCollection>
        <dx:PopupControlContentControl ID="PopupControlContentControl3" runat="server">
            <dx:ASPxGridView ID="gvPendingApprovalList" ClientInstanceName="gvPendingApprovalList" Width="100%"
                KeyFieldName="PlanId" runat="server" AutoGenerateColumns="false" DataSourceID="odsPendingApprovalList"
                 SettingsBehavior-AllowSelectSingleRowOnly="true">
                <Columns>
                    <dx:GridViewCommandColumn ShowSelectCheckbox="True" VisibleIndex="0">
                    </dx:GridViewCommandColumn>

                    <dx:GridViewDataTextColumn FieldName="EmployeeName" Caption="Employee Name" Width="10%" VisibleIndex="1"></dx:GridViewDataTextColumn>
                    <dx:GridViewDataDateColumn FieldName="Month" Caption="Month" SortOrder="Descending" Width="10%" VisibleIndex="2"></dx:GridViewDataDateColumn>
                    <dx:GridViewDataTextColumn FieldName="Year" Caption="Year" Width="20%" VisibleIndex="3"></dx:GridViewDataTextColumn>
                    <dx:GridViewDataTextColumn FieldName="ApproveStatus" Caption="Submitted" VisibleIndex="3"></dx:GridViewDataTextColumn>
                </Columns>
                <SettingsPager PageSize="5">
                </SettingsPager>
                <Settings ShowStatusBar="Hidden" ShowFilterRow="false" ShowGroupPanel="false" />
                <SettingsDataSecurity AllowDelete="False" AllowEdit="False" AllowInsert="False" />
            </dx:ASPxGridView>
            <asp:ObjectDataSource ID="odsPendingApprovalList" runat="server" TypeName="Assistpro.AssistproCRM.Presenter.PlanPresenter"
                SelectMethod="GetDirectReporteesSubmittedPlansByUserId" OnObjectCreating="odsPendingApprovalList_ObjectCreating"></asp:ObjectDataSource>
            <input type="button" name="btnLoadMe" id="btnLoadMe" value=" Load Me " />
            <dx:ASPxButton ID="btnLoad" runat="server" Text="Load Plan" AutoPostBack="false">
                <ClientSideEvents Click="checkMe(this)" />
            </dx:ASPxButton>
            <dx:ASPxButton ID="btnCancelList" runat="server" Text="Cancel">
                <ClientSideEvents Click="CancelPendingApprovalList" />
            </dx:ASPxButton>
        </dx:PopupControlContentControl>


    </ContentCollection>


</dx:ASPxPopupControl>

<dx:ASPxCallback ID="cbPendingApprovals" ClientInstanceName="cbPendingApprovals" OnCallback="cbPendingApprovals_Callback" runat="server">
    <ClientSideEvents  />
</dx:ASPxCallback>

protected void cbPendingApprovals_Callback(object source, DevExpress.Web.CallbackEventArgs e)
       {
           List<string> fieldNames = new List<string>();
           foreach (GridViewColumn column in gvPendingApprovalList.Columns)
               if (column is GridViewDataColumn)
                   fieldNames.Add(((GridViewDataColumn)column).FieldName);
           List<object> selectedValues = gvPendingApprovalList.GetSelectedFieldValues(fieldNames.ToArray());
       }

but selectedvalues i am getting is 0 items.

Please help