For core22 projects, validation errors from `Project.unmarshal()` in the classic lifecycle weren't handled, so any invalid value in `snapcraft.yaml`, such as `grade: foo`, was shown as a pydantic traceback. These errors are now raised as `ProjectValidationError` and formatted with craft-application's `format_pydantic_errors()`, so they look the same as for core24 and newer projects: ``` Bad snapcraft.yaml content: - input should be 'stable' or 'devel' (in field 'has-base.core22.grade', input: 'foo') ``` Grammar processing also crashed with `TypeError: 'NoneType' object is not iterable` on keys without a value, like the empty root-level `build-packages:` from the issue. These keys are now left for the project model to validate: ``` Bad snapcraft.yaml content: - input should be a valid list (in field 'has-base.core22.parts.snapcraft/core.build-packages', input: None) ``` An empty part-level `stage-packages:` still fails with an internal error, on core24 as well, because craft-parts' `PartSpec` validator iterates over the value before type validation. That needs a separate fix in craft-parts. Fixes #5613 --- - [x] I've followed the [contribution guidelines](https://github.com/canonical/snapcraft/blob/main/CONTRIBUTING.md). - [x] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [x] I've successfully run `make lint && make test`. - [ ] I've added or updated any relevant documentation. - [ ] In documents I changed, I [added a meta description](https://canonical-starflow.readthedocs-hosted.com/how-to/add-a-page-meta-description/) if one was missing. - [ ] I've updated the relevant release notes.