code: def test_add_null(self):
      ds = DataSet()
      with pytest.raises(RuntimeError) as RE:
          ds.add_field("test", [])
  
imports: import os
  import pytest
  import numpy
  from fastNLP.core.dataset import DataSet, FieldArray, Instance, ApplyResultException
  from fastNLP import logger
label_desc: Use image node_id 25 for calling the TestDataSetMethods obj's underlying member method code with example usage: obj.test_add_null() without return types