loding row event not working when datagrid is hidden

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WPF

DataGrid

event

hi
I place Data grid in popup control in wpf
and i Added loding row event to datagrid to calculate one column Sum

Its working fine when popup is visible but it not calculating when popup is hidden
Please help me to solve this issue
Iam using c# in back end

<br />
<Popup Name="poppurchaseorder"<br />
                   VerticalOffset="20"<br />
                   PlacementRectangle="300,140,100,150"<br />
                   PopupAnimation="Fade"<br />
                   AllowsTransparency="True"<br />
                   Margin="300,100,-127,-254"<br />
                   Placement="Center"<br />
                   IsOpen="False"><br />
<br />
<DataGrid Style="{StaticResource DataGrid}"<br />
                              IsReadOnly="True"<br />
                              HorizontalAlignment="Left" <br />
                              Margin="70,103,0,0"<br />
                              VerticalAlignment="Top"<br />
                              Width="390" Height="217" Name="grdadditionaltransaction"<br />
                              SelectionChanged="grdadditionaltransaction_SelectionChanged_1"<br />
                              LoadingRow="grdadditionaltransaction_LoadingRow_1"/><br />
 </Popup><br />
<br />