@Override
    public ICachedMNode next() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      ICachedMNode result = nextSubtree;
      nextSubtree = null;
      return result;
    }
Collect updated storage group node.

@return null if not exist
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/schemaengine/schemaregion/mtree/impl/pbtree/memory/MemoryManager.java