### Bug Description If something horrifically unexpected happens in a YAML snapcraft will throw a traceback instead of reporting a proper error. Specifically, snapcraft will usefully tell me that I have some bad snapcraft.yaml content and what was expected when building `base: core24` snaps. However, a traceback is thrown instead of I use `base: core22` snaps. ### To Reproduce Install snapcraft 8.x (using 8.10.y myself) and: ``` mkdir t && cd t; snapcraft init echo 'build-packages:' >> snap/snapcraft.yaml snapcraft sed -i 's/core24/core22/' snap/snapcraft.yaml snapcraft ``` ### Environment Ubuntu 25.04, snapcraft 8.x, building with LXD. ### snapcraft.yaml ```yaml name: test # you probably want to 'snapcraft register <name>' base: core22 # the base snap is the execution environment for this snap version: '0.1' # just for humans, typically '1.2+git' or '1.3.2' summary: Single-line elevator pitch for your amazing snap # 79 char long summary description: | This is my-snap's description. You have a paragraph or two to tell the most important story about your snap. Keep it under 100 words though, we live in tweetspace and your description wants to look good in the snap store. grade: devel # must be 'stable' to release into candidate/stable channels confinement: devmode # use 'strict' once you have the right plugs and slots parts: my-part: # See 'snapcraft plugins' plugin: nil build-packages: ``` ### Relevant log output ```shell dilyn@Ares:~/test -> sc Starting snapcraft, version 8.10.0.post45 Logging execution to '/home/dilyn/.local/state/snapcraft/log/snapcraft-20250714-150700.168675.log' Bad snapcraft.yaml content: - input should be a valid list (in field 'parts.snapcraft/core.build-packages') dilyn@Ares:~/test -> hx snap/snapcraft.yaml dilyn@Ares:~/test -> sc Starting snapcraft, version 8.10.0.post45 Logging execution to '/home/dilyn/.local/state/snapcraft/log/snapcraft-20250714-150707.652601.log' Running on amd64 for amd64Traceback (most recent call last): File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/application.py", line 467, in main return app.run() ^^^^^^^^^ File "/snap/snapcraft/x19/lib/python3.12/site-packages/craft_application/application.py", line 615, in run return_code = self._run_inner() ^^^^^^^^^^^^^^^^^ File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/application.py", line 178, in _run_inner return_code = super()._run_inner() ^^^^^^^^^^^^^^^^^^^^ File "/snap/snapcraft/x19/lib/python3.12/site-packages/craft_application/application.py", line 561, in _run_inner dispatcher = self._get_dispatcher() ^^^^^^^^^^^^^^^^^^^^^^ File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/application.py", line 405, in _get_dispatcher self._check_for_classic_fallback() File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/application.py", line 317, in _check_for_classic_fallback raise errors.ClassicFallback() snapcraft.errors.ClassicFallback During handling of the above exception, another exception occurred: Traceback (most recent call last): File "/snap/snapcraft/x19/bin/snapcraft", line 8, in <module> sys.exit(main()) ^^^^^^ File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/application.py", line 470, in main return cli.run() ^^^^^^^^^ File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/cli.py", line 256, in run _run_dispatcher(dispatcher, global_args) File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/cli.py", line 229, in _run_dispatcher dispatcher.run() File "/snap/snapcraft/x19/lib/python3.12/site-packages/craft_cli/dispatcher.py", line 528, in run return self._loaded_command.run(self._parsed_command_args) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/commands/core22/lifecycle.py", line 272, in run super().run(parsed_args) File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/commands/core22/lifecycle.py", line 145, in run parts_lifecycle.run(self.name, parsed_args) File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/parts/lifecycle.py", line 95, in run yaml_data_for_arch = yaml_utils.apply_yaml(yaml_data, build_on, build_for) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/parts/yaml_utils.py", line 203, in apply_yaml yaml_data["parts"] = grammar.process_parts( ^^^^^^^^^^^^^^^^^^^^^^ File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/parts/grammar.py", line 77, in process_parts process_part(part_yaml_data=part, processor=processor) File "/snap/snapcraft/x19/lib/python3.12/site-packages/snapcraft/parts/grammar.py", line 47, in process_part processed_grammar: list[Any] | None = processor.process( ^^^^^^^^^^^^^^^^^^ File "/snap/snapcraft/x19/lib/python3.12/site-packages/craft_grammar/_processor.py", line 89, in process for section in grammar: TypeError: 'NoneType' object is not iterable ``` ### Additional context _No response_