https://github.com/10gen/mongo/blob/f314b9690b1677033e17b0b2f56590c2f29636b4/src/mongo/db/s/migration_chunk_cloner_source.cpp#L1212

 _sessionCatalogSoure is currently only initialized when mode is in repl set. Since we are going to make repl set enabled all the time we can probably get rid of the conditional initialization. For older branches, we should add if condition to check if it is null like the rest of the code.