ValueChange null for second selection of same row

I have a Table that displays the information from a db table (MySQL) - using TableQuery.

Following the example from the Vaadin book, I want to display some information when a row is clicked, so I use Propery.ValueChangeListener.

My problem is that, when a row is selected for the second time, I have a null value returned by the event.

So, step 1 user select row 1 → everything is OK.
If, in a second step, the user selects again the same row, event is null.