code: private FieldExpression ensureInstance(final FieldExpression expression, final FieldExpression defaultExpression) {
          Preconditions.checkNotNull(defaultExpression, "Default expression must not be null");
          if (expression != null) {
              return expression;
          } else {
              return defaultExpression;
          }
      }
docs: Give an expression instance, will return it if is not null. Otherwise will return the defaultExpression;
  
  @param expression        - CronFieldExpression instance; may be null
  @param defaultExpression - CronFieldExpression, never null;
  @return the given expression or the given defaultExpression in case the given expression is {@code null}