Faceted Search > Coverage and Subjects need the link to open the selection pop up

The coverage and subject fields in the Categorization widget have a '+' link to open a pop up, with a tree to select the categorization facet.
For now, an empty choice is displayed, you need to delete it to get the + link to open the pop up and make your selection.
This selected 'null' choice is disturbing, the + link has to be displayed first as you didn't select any filter.