code: def testExtractSubGraphWithInvalidDestNodes(self):
      graph_def = graph_pb2.GraphDef()
      n1 = graph_def.node.add()
      n1.name = "n1"
      with self.assertRaisesRegex(TypeError, "must be an iterable"):
          graph_util.extract_sub_graph(graph_def, "n1")
  
imports: from tensorflow.core.framework import attr_value_pb2
  from tensorflow.core.framework import function_pb2
  from tensorflow.core.framework import graph_pb2
  from tensorflow.core.framework import node_def_pb2
  from tensorflow.python.framework import constant_op
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import function
  from tensorflow.python.framework import graph_util
  from tensorflow.python.framework import ops
  from tensorflow.python.framework import tensor_util
  from tensorflow.python.framework import test_util
  from tensorflow.python.ops import gen_state_ops
  from tensorflow.python.ops import math_ops
  from tensorflow.python.ops import variable_v1
  from tensorflow.python.platform import test
  from tensorflow.python.util import compat
label_desc: Use image node_id 6 for calling the GraphUtilTest obj's underlying member method code with example usage: obj.testExtractSubGraphWithInvalidDestNodes() without return types