Frederic, Looking at the documentation list widget has property SelectedText that is field value that is currently selected and SelectedItems that is defined as infotable field that is currently selected. Have you tried unsetting the SelectedItems instead of SelectedText?
Hi thank you for recommending this,
I've tried it by adding this line or code (I've tried two way) :
1- thisWidget.setProperty( 'SelectedItems',  );
2- thisWidget.setProperty( 'SelectedItems', undefined );
None of these lines worked, maybe I'm doing something wrong or maybe selectedItems doesn't work that way.