SOLR-5311 was done without full understanding of the system and must be reverted.

In class org.apache.solr.cloud.Overseer the Line 360:
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if (sliceName !=null && collectionExists && !"true".equals(state.getCollection(collection).getStr("autoCreated"))) {
Slice slice = state.getSlice(collection, sliceName);
if (slice.getReplica(coreNodeName) == null)

{ log.info("core_deleted . Just return"); return state; }
}
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the slice needs to be checked null .because when create a new core with both explicite shard and coreNodeName, the state.getSlice(collection, sliceName) may return a null.So it needs to be checked ,or there will be an NullpointException
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if (sliceName !=null && collectionExists && !"true".equals(state.getCollection(collection).getStr("autoCreated"))) {
Slice slice = state.getSlice(collection, sliceName);
if (slice != null && slice.getReplica(coreNodeName) == null) { log.info("core_deleted . Just return"); return state; }

}
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