Cause of 'Cannot run with sound null safety' Error
- This error typically occurs because some dependencies in the Flutter project do not support sound null safety. Null safety is a feature introduced in Dart to help control nullable and non-nullable types, thus improving code quality and reducing runtime errors. If a package has not been updated to include null safety, it can cause conflicts when attempting to run a Flutter project in a null-safe environment.
- Some older packages or libraries might not have been maintained or upgraded to support null safety, causing projects using those dependencies to fail when running in a null-safe environment.
- Mixing null-safe enabled and non-null-safe libraries within a project leads to this issue. This happens because when the Flutter application attempts to enable sound null safety, it requires that all the dependencies also support null safety.
Dependency Version Constraints
- If the version constraints in the `pubspec.yaml` file are set to outdated versions of the dependencies, it could inadvertently enforce using versions of libraries that are not null-safe. It is essential to ensure that the constraints are set to versions that include null safety, albeit such measures might require manual adjustments.
- Another cause of this issue might be transitive dependencies, where a package used by a dependency within the project does not support null safety, even if the direct dependency does. This can be seen when an updated package depends on another package that has yet to adopt null safety.
Example of Dependency Declaration
dependencies:
flutter:
sdk: flutter
some_nullable_package: ^0.1.2
- Here, `some_nullable_package` might not have null safety, indicated by a version that predates a null-safe update. This would cause an error when attempting to run with sound null safety.
Project Downgrade
- Sometimes, the Flutter SDK itself or Dart SDK might be in a version that assumes non-null-safe libraries by default. While this is uncommon, particularly if the SDK is not updated regularly, it can lead to conflicts and the error message in question.
- Similarly, to save time and resources, developers might intentionally use older versions of the SDK to maintain compatibility with various tools or plugins, resulting in constraints against null safety utilization.
