code: private void assertFieldValue(Field field, Object fieldValue) {
      if (fieldValue == null && isFieldRequired(field)) {
        throw new IllegalArgumentException("Non-nullable field " + field.getName()
            + ". This field is either annotated as REQUIRED or is a primitive type.");
      }
    }
docs: Asserts that a field annotated as {@link BigQueryFieldMode#REPEATED} is not left null.