code: def test_getitem_missing_nested(self):
      f = FlatMap({"~": {}})
      with self.assertRaises(KeyError):
          f["~"]["missing"]
  
imports: import collections
  import copy
  import pickle
  import threading
  from absl.testing import absltest
  from absl.testing import parameterized
  import cloudpickle
  import dill
  from haiku._src import data_structures
  import jax
  import tree
label_desc: Use image node_id 6 for calling the FlatMappingTest obj's underlying member method code with example usage: obj.test_getitem_missing_nested() without return types