Fix compatibility with Hierarchy Manager

Thanks for reporting the issue and providing a patch. I reproduced and confirm it fixes the fatal error. However, I feel flat_taxonomy and hierarchy_manager are kind of incompatible. HM is also overriding the UI and so it is possible make taxonomies nested despite flat_taxonomy. On vocabulary load, the vocabulary is flatten again. I will add a warning in admin/reports if flat_taxonomy and hierarchy_manager are configured on the same taxonomy.

Automatically closed - issue fixed for 2 weeks with no activity.