[Doris On ES] All DOE table sync meta-data failure after upgrading FE #3960

Step to Repoduce:

1. create ES external Table and start Doris FE
2. After 1min or more, restart FE
3. All DOE table sync meta-data failure

2020-06-28 10:57:27,429 WARN 37 [EsTable.syncESIndexMeta():429] Exception happens when fetch index [wyf_test] meta data from remote es cluster
java.lang.NullPointerException: null
at org.apache.doris.catalog.EsTable.syncESIndexMeta(EsTable.java:416) [palo-fe.jar:?]
at org.apache.doris.external.elasticsearch.EsRepository.runAfterCatalogReady(EsRepository.java:73) [palo-fe.jar:?]
at org.apache.doris.common.util.MasterDaemon.runOneCycle(MasterDaemon.java:58) [palo-fe.jar:?]
at org.apache.doris.common.util.Daemon.run(Daemon.java:116) [palo-fe.jar:?]
2020-06-28 10:57:27,431 WARN 37 [EsTable.syncESIndexMeta():429] Exception happens when fetch index [wyf_test_docvalue] meta data from remote es cluster
java.lang.NullPointerException: null
at org.apache.doris.catalog.EsTable.syncESIndexMeta(EsTable.java:416) [palo-fe.jar:?]
at org.apache.doris.external.elasticsearch.EsRepository.runAfterCatalogReady(EsRepository.java:73) [palo-fe.jar:?]
at org.apache.doris.common.util.MasterDaemon.runOneCycle(MasterDaemon.java:58) [palo-fe.jar:?]
at org.apache.doris.common.util.Daemon.run(Daemon.java:116) [palo-fe.jar:?]
2020-06-28 10:57:27,433 WARN 37 [EsTable.syncESIndexMeta():429] Exception happens when fetch index [case_replay_for_milimin] meta data from remote es cluster
java.lang.NullPointerException: null
at org.apache.doris.catalog.EsTable.syncESIndexMeta(EsTable.java:416) [palo-fe.jar:?]
at org.apache.doris.external.elasticsearch.EsRepository.runAfterCatalogReady(EsRepository.java:73) [palo-fe.jar:?]
at org.apache.doris.common.util.MasterDaemon.runOneCycle(MasterDaemon.java:58) [palo-fe.jar:?]
at org.apache.doris.common.util.Daemon.run(Daemon.java:116) [palo-fe.jar:?]