Grid select row in Selection Mode MULTI not highlighted

i have a grid with SelectionMode MULTI. When i click a row i can fetch the item with a addItemClickListener but the row is not hightlighted. So i would like to add it programatically, but get an Nullpointer when doing:

 grid.addItemClickListener((selectionEvent -> productSelected(selectionEvent)));
 //more Code
  private void productSelected(ItemClickEvent<Product> selectionEvent) {
        grid.getSelectionModel().getSelectedItems().add(selectionEvent.getItem()); //getSelectedItems = NULL

How can i archive it: got multi select enabled, but a single line Click should also add the row to the selected items?

There is example code in our Cookbook for similar case, you could take a look at that: