Minor Description
when retrieving user/group by id or tree the user mgr implementation will verify that the id associated with a given tree actually is of type authorizable and return null if that's not the case. consequently, line 437 in UserManagerImpl will never be reached.
stillalex, i will refactor the code such that the ID lookup is limited to one single place in getAuthorizable(Tree) and drop the extra if, which will not be reached.