code: public List<Variant> read() {
          Variant variant = null;
          boolean valid = false;
          while (iterator.hasNext() && !valid) {
              variant = iterator.next();
              valid = isValid(variant);
          }
          if (valid) {
              // New variants in the variation collection created during the update of the frequencies may not have
              // the variant type set and this might cause NPE
              if (variant.getType() == null) {
                  variant.setType(variant.inferType(variant.getReference(), variant.getAlternate()));
              }
              return Collections.singletonList(variant);
          } else {
              return null;
          }
      }
docs: Performs one read of from a CellBase variation collection.
  
  @return  List of variants. It can be expected to contain only one variant.