[Backport to 7.1.1] investigate possibility of indexer crash during rebalance index movement

MB-51570: Indexer crash with error "panic: Fail to find real index instance 3055985754217342362 we observed realIndexInst was being deleted before proxy index build was complete which caused panic, this issue has been fixed but similar case may also happen if, while the proxy instance is building, the real instance gets moved to another node due to next transfer token publish. And when the proxy is done building, it finds there is no real instance. This MB is to investigate if there is protection in the code for that situation.