containerAdapter.setItemLabelConverter(new Converter<Container, String>() {
         @Override
         public String convert(Container source) {
            return source.getId();
         }
      });

In Eclipse, the Converter is called with a "null" argument during Step/Command initialization. Unless the user specifically checks for null a NullPointerException is thrown.

This is especially odd in the case where the user provide a Set of ValueChoices to convert and none of those are null.