When we make monitor rule, something failure happen

Currently we are constructing the following environment.

And when we make monitor rule, the following error happen.
Does anyone meet the same situation??

[2019-12-02T16:19:25,240][ERROR][c.a.o.a.MonitorRunner ] [elasticsearch-cordinating-only-node-host] Error loading alerts for monitor: na
org.elasticsearch.transport.RemoteTransportException: [elasticsearch-master-node-host][100.73.150.53:9300][indices:admin/create]
Caused by: org.elasticsearch.index.mapper.MapperParsingException: Failed to parse mapping [_doc]: Root mapping definition has unsupported parameters: [_doc : {}]
at org.elasticsearch.index.mapper.MapperService.internalMerge(MapperService.java:398) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.index.mapper.MapperService.merge(MapperService.java:327) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.metadata.MetaDataCreateIndexService$IndexCreationTask.execute(MetaDataCreateIndexService.java:476) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.ClusterStateUpdateTask.execute(ClusterStateUpdateTask.java:47) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.MasterService.executeTasks(MasterService.java:687) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.MasterService.calculateTaskOutputs(MasterService.java:310) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.MasterService.runTasks(MasterService.java:210) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.MasterService$Batcher.run(MasterService.java:142) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.TaskBatcher.runIfNotProcessed(TaskBatcher.java:150) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.TaskBatcher$BatchedTask.run(TaskBatcher.java:188) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:688) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:252) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:215) ~[elasticsearch-7.2.0.jar:7.2.0]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[?:?]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[?:?]
at java.lang.Thread.run(Thread.java:835) ~[?:?]
Caused by: org.elasticsearch.index.mapper.MapperParsingException: Root mapping definition has unsupported parameters: [_doc : {}]
at org.elasticsearch.index.mapper.DocumentMapperParser.checkNoRemainingFields(DocumentMapperParser.java:148) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.index.mapper.DocumentMapperParser.parse(DocumentMapperParser.java:136) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.index.mapper.DocumentMapperParser.parse(DocumentMapperParser.java:88) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.index.mapper.MapperService.internalMerge(MapperService.java:396) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.index.mapper.MapperService.merge(MapperService.java:327) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.metadata.MetaDataCreateIndexService$IndexCreationTask.execute(MetaDataCreateIndexService.java:476) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.ClusterStateUpdateTask.execute(ClusterStateUpdateTask.java:47) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.MasterService.executeTasks(MasterService.java:687) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.MasterService.calculateTaskOutputs(MasterService.java:310) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.MasterService.runTasks(MasterService.java:210) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.MasterService$Batcher.run(MasterService.java:142) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.TaskBatcher.runIfNotProcessed(TaskBatcher.java:150) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.cluster.service.TaskBatcher$BatchedTask.run(TaskBatcher.java:188) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:688) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:252) ~[elasticsearch-7.2.0.jar:7.2.0]
at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:215) ~[elasticsearch-7.2.0.jar:7.2.0]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[?:?]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[?:?]
at java.lang.Thread.run(Thread.java:835) ~[?:?]

I could realize cause of this issue.
Somehow the following default mapping template was cause of this issue…

{
   "order": -1,
   "index_patterns": "*",
   "settings": {
     "index": {
         "refresh_interval": "30s",
         "store": {
           "type": "niofs"
         },
      "number_of_shards": "5",
      "number_of_replicas": "1"
     }
   },
   "mappings": {},
   "aliases": {}
}

After deleting this mapping template, issue was fixed somehow lol
Does anyone can know why this issue was happened???