It looks like OH9360.ARE doesn't contain a rest encounter structure (rest encounter offset is 0). fj_are_structure seems to set the rest offset to a non-zero value without adding a rest encounter structure. As a result NI attempts to read non-existing data which triggers an error.
I can probably avoid this error by adding more checks, but that's only a hack. Ideally fj_are_structure should be fixed to ensure that offsets pointing to non-existing single structure elements (such as songs or rest encounters) are set to zero.
Maybe not worth it. I assume it is something unique to those Black Pit areas. It sems to have no real impact on the gameplay as such. The only issue I found was in NI when I tried to quickly look up something during mod creation. Unless you think it is a more widespread problem. I have used the same fl_are technique to add exit or entry to numerous original areas and there is never any problem.