Problem in saving persistant order book due to concurrency issues

[maryarm]

Caused by: java.util.ConcurrentModificationException: null
at java.util.TreeMap$PrivateEntryIterator.nextEntry(TreeMap.java:1211) ~[na:1.8.0_212]
at java.util.TreeMap$ValueIterator.next(TreeMap.java:1256) ~[na:1.8.0_212]
at co.nilin.opex.matching.core.engine.SimpleOrderBook.persistent(SimpleOrderBook.kt:468)