I don't think this is the solution.  From what I know of how this code works, we shouldn't be here for a directory name.  If we are and i.getDirCacheEntry() == null, we would execute the else case below and call keep(null), which should also then NPE, or NPE very late downstream when the new index contains an empty slot.

I think the better question is, why are we looking at a subtree here in "i" and in this code path?