Finally based on above discussions if you have to set IsLightDismissEnabled = false and want to close popup by clicking somewhere outside you can:
1) Create follow XAML code:
<Page>
<Grid x:Name="OuterGrid">
<Grid x:Name="InnerGrid">
// all other content
</Grid>
</Grid>
</Page>
2) When popup is opened - set IsHitTestVisible = False for InnerGrid and subscribe to Tapped event of OuterGrid (and inside of event handler you need to close popup)
3) When popup is closed - set IsHitTestVisible = True for InnerGrid and unsubscribe from Tapped event of OuterGrid.