hardCause property
The hard cause is some error which caused the UserException, but that is not
a UserException itself. For example: int.parse('a') throws a FormatException.
Then: throw UserException('Invalid number').addCause(FormatException('Invalid input')).
will have the FormatException as the hard cause. Note: If a UserException is
passed as the hard cause, it will be added with addCause, and will not become the
hard cause. In other words, a UserException will never be a hard cause.
Implementation
final Object? hardCause;