OK, thanks for confirming the stacktrace. You can report this behavior as an issue on https://github.com/JanusGraph/janusgraph/issues, referring to this thread. It is still not clear to me how this exception can occur, because the BigTable compatibility layer reuses the HBase backend, for which graph.getBackend().getStoreManager().getHadoopManager(); is available.

So, I am afraid there is no quick fix for your issue, unless you start debugging MapReduceIndexManagement for BigTable yourself. Maybe, simply reloading the graph is an option.

Best wishes,    Marc