Alaeddine,
In the SearchWidget.mxml the showHighlight function is what displays the popup.
Don't forget to click the Mark as answer check on this post and to click the top arrow (promote).
Follow the steps as shown in the below graphic:
Alaeddine,
Do you have a popup configured for that particular layer in your main config.xml?
graphic.infoWindowRenderer = infoWindowRenderer;
Alaeddine,
Search for and comment out this line then:graphic.infoWindowRenderer = infoWindowRenderer;