@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