In Importacular version 4.9 a change was made to how Importacular validates gifts as they are imported into batches. The code tries to verify that the required gift attribute is present as this is being added to the Batch and since the attribute can't be imported until the gift is import, the result is an exception.
This will be resolved for versions after 4.9.0.1, but the workaround is to uncheck the Required field for the gift attribute prior to importing and then re-checking the Required box after the import is complete.