Indexing errors: "Undefined property" and "Array to string conversion"

Hi, I'm relatively new to Drupal and got the following errors when installing the Views Natural Sort module:

Notice: Array to string conversion in views_natural_sort_views_natural_sort_get_rebuild_data() (line 103 of .../sites/all/modules/views_natural_sort/views_natural_sort.module).
Notice: Undefined property: OgMembershipType::$Array in views_natural_sort_views_natural_sort_get_rebuild_data() (line 103 of .../sites/all/modules/views_natural_sort/views_natural_sort.module).

Notice: Array to string conversion in views_natural_sort_views_natural_sort_get_rebuild_data() (line 103 of .../sites/all/modules/views_natural_sort/views_natural_sort.module).
Notice: Undefined property: Profile::$Array in views_natural_sort_views_natural_sort_get_rebuild_data() (line 103 of .../sites/all/modules/views_natural_sort/views_natural_sort.module).

Notice: Array to string conversion in views_natural_sort_views_natural_sort_get_rebuild_data() (line 103 of .../sites/all/modules/views_natural_sort/views_natural_sort.module).
Notice: Undefined property: RulesReactionRule::$Array in views_natural_sort_views_natural_sort_get_rebuild_data() (line 103 of .../sites/all/modules/views_natural_sort/views_natural_sort.module).

Notice: Array to string conversion in views_natural_sort_views_natural_sort_get_rebuild_data() (line 103 of .../sites/all/modules/views_natural_sort/views_natural_sort.module).
Notice: Undefined property: stdClass::$Array in views_natural_sort_views_natural_sort_get_rebuild_data() (line 103 of .../sites/all/modules/views_natural_sort/views_natural_sort.module).

I searched existing issues for this problem, but could not find a solution. On one thread someone mentioned that the "file entity" module might be causing problems, so I disabled and uninstalled that module, then reinstalled Views Natural Sort, but I still got the same errors.... Any ideas for how to solve this? Thank you!