ERROR - An error occurred while returning credits for client and NPE

Issue:

NPE statement in hivemq.log:

ERROR  - An error occurred while returning credits for client '***********'.
java.lang.NullPointerException: null
	at hmq.hk.a.a(Source.java:142)
	at hmq.hk.a$$Lambda$1206/00000000780049D0.run(Unknown Source)
	at io.netty.util.concurrent.AbstractEventExecutor.runTask(Source.java:174)
	at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(Source.java:167)
	at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(Source.java:470)
	at io.netty.channel.epoll.EpollEventLoop.run(Source.java:391)
	at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(Source.java:997)
	at io.netty.util.internal.ThreadExecutorMap$2.run(Source.java:74)
	at java.base/java.lang.Thread.run(Thread.java:834)

Version:

4.9.1

Severity:
High (refill of the credits stops, no client will get its credits back)

Fix:

Upgrade to 4.9.2

Page:
ERROR - An error occurred while returning credits for client and NPE (HiveMQ Knowledge Base - Self Managed Offering)