code: def _has_nulltype(dt):
      
      if isinstance(dt, StructType):
          return any(_has_nulltype(f.dataType) for f in dt.fields)
      elif isinstance(dt, ArrayType):
          return _has_nulltype((dt.elementType))
      elif isinstance(dt, MapType):
          return _has_nulltype(dt.keyType) or _has_nulltype(dt.valueType)
      else:
          return isinstance(dt, NullType)
docs: Return whether there is NullType in `dt` or not