Assuming the files_tools plugin on your system is compatible with 1.8, this error would likely be caused by a file that has been deleted or is not accessible by the user requesting it. What were you doing when we see the error? Are you logged in as an admin?

I guess it might be an access level issue, possibly connected with a Files entry within a (restricted?) group. There was an issue in older Elgg versions (as it seems at least up to Elgg 1.8) where it could happen that an entity itself was visible (or stuff like a river entry connected with this entity) but due to a more restricted access level of the container entity of this entity (e.g. container could be a group) accessing the entity itself failed.

I'm not 100% sure this is the case here but I suspect it is. Only solution then would be to upgrade to a newer Elgg version than 1.8 where this problem got fixed (hiding everything of an entity if the user lacks the necessary permissions to access the container entity).