TypeError: argument of type 'NoneType' is not iterable when testing core

Trying to run rasa test core --fail-on-prediction-errors --out results/results-core-test, I receive this error:

Traceback (most recent call last):
  File "/opt/venv/bin/rasa", line 8, in <module>
    sys.exit(main())
  File "/opt/venv/lib/python3.8/site-packages/rasa/__main__.py", line 117, in main
    cmdline_arguments.func(cmdline_arguments)
  File "/opt/venv/lib/python3.8/site-packages/rasa/cli/test.py", line 133, in run_core_test
    test_core(
  File "/opt/venv/lib/python3.8/site-packages/rasa/model_testing.py", line 182, in test_core
    rasa.utils.common.run_in_loop(
  File "/opt/venv/lib/python3.8/site-packages/rasa/utils/common.py", line 296, in run_in_loop
    result = loop.run_until_complete(f)
  File "uvloop/loop.pyx", line 1456, in uvloop.loop.Loop.run_until_complete
  File "/opt/venv/lib/python3.8/site-packages/rasa/core/test.py", line 1041, in test
    story_evaluation, _, entity_results = await _collect_story_predictions(
  File "/opt/venv/lib/python3.8/site-packages/rasa/core/test.py", line 916, in _collect_story_predictions
    ) = await _predict_tracker_actions(
  File "/opt/venv/lib/python3.8/site-packages/rasa/core/test.py", line 785, in _predict_tracker_actions
    ) = _collect_action_executed_predictions(
  File "/opt/venv/lib/python3.8/site-packages/rasa/core/test.py", line 704, in _collect_action_executed_predictions
    if FormPolicy.__name__ in prediction.policy_name:
TypeError: argument of type 'NoneType' is not iterable

I don’t know if it’s a bug or how to debug, if someone could help I appreciate.

Does rasa data validate report any issues?

Running that command I get:

Traceback (most recent call last):
  File "/opt/venv/lib/python3.8/site-packages/rasa/shared/core/training_data/loading.py", line 113, in load_data_from_files
    steps = reader.read_from_file(story_file)
  File "/opt/venv/lib/python3.8/site-packages/rasa/shared/core/training_data/story_reader/yaml_story_reader.py", line 99, in read_from_file
    return self.read_from_string(
  File "/opt/venv/lib/python3.8/site-packages/rasa/shared/core/training_data/story_reader/yaml_story_reader.py", line 127, in read_from_string
    return self.read_from_parsed_yaml(yaml_content)
  File "/opt/venv/lib/python3.8/site-packages/rasa/shared/core/training_data/story_reader/yaml_story_reader.py", line 152, in read_from_parsed_yaml
    parser.parse_data(data)
  File "/opt/venv/lib/python3.8/site-packages/rasa/shared/core/training_data/story_reader/yaml_story_reader.py", line 220, in parse_data
    self._parse_plain_item(item)
  File "/opt/venv/lib/python3.8/site-packages/rasa/shared/core/training_data/story_reader/yaml_story_reader.py", line 250, in _parse_plain_item
    self._parse_step(step)
  File "/opt/venv/lib/python3.8/site-packages/rasa/shared/core/training_data/story_reader/yaml_story_reader.py", line 283, in _parse_step
    self._parse_slot(step)
  File "/opt/venv/lib/python3.8/site-packages/rasa/shared/core/training_data/story_reader/yaml_story_reader.py", line 478, in _parse_slot
    SlotSet.type_name, {slot: self._slot_default_value(slot)}
  File "/opt/venv/lib/python3.8/site-packages/rasa/shared/core/training_data/story_reader/yaml_story_reader.py", line 499, in _slot_default_value
    slot = next(slot for slot in self.domain.slots if slot.name == slot_name)
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/opt/venv/bin/rasa", line 8, in <module>
    sys.exit(main())
  File "/opt/venv/lib/python3.8/site-packages/rasa/__main__.py", line 117, in main
    cmdline_arguments.func(cmdline_arguments)
  File "/opt/venv/lib/python3.8/site-packages/rasa/cli/data.py", line 248, in validate_files
    validator = rasa.utils.common.run_in_loop(Validator.from_importer(file_importer))
  File "/opt/venv/lib/python3.8/site-packages/rasa/utils/common.py", line 296, in run_in_loop
    result = loop.run_until_complete(f)
  File "uvloop/loop.pyx", line 1456, in uvloop.loop.Loop.run_until_complete
  File "/opt/venv/lib/python3.8/site-packages/rasa/validator.py", line 52, in from_importer
    story_graph = await importer.get_stories()
  File "/opt/venv/lib/python3.8/site-packages/rasa/shared/importers/rasa.py", line 53, in get_stories
    return await utils.story_graph_from_paths(
  File "/opt/venv/lib/python3.8/site-packages/rasa/shared/importers/utils.py", line 25, in story_graph_from_paths
    story_steps = await loading.load_data_from_files(
RuntimeError: coroutine raised StopIteration

Take a look at this issue. Is your domain.yml in root and your training data in /data? You may need to use the command line options if you have a custom repo setup.