Notice: Undefined offset: 0 in _taxonomy_access_form_extract_entity() on user add and edit form

If there are taxonomy fields on a user entity, TAC triggers the following notice on the user add and edit forms:

Notice: Undefined offset: 0 in _taxonomy_access_form_extract_entity()

This issue may be resolved by using #1204230: Missing hook_field_widget_form_alter() instead of the generic hook_form_alter().