java.util.ConcurrentModificationException: This exception has no message.

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java.util.ConcurrentModificationException:
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1437)
at java.util.HashMap$KeyIterator.next(HashMap.java:1461)
at org.mapdb.SerializerBase.serializeMap(SerializerBase.java:818)
at org.mapdb.SerializerBase.serialize2(SerializerBase.java:377)
at org.mapdb.SerializerBase.serialize(SerializerBase.java:150)
at org.mapdb.SerializerBase.serialize(SerializerBase.java:94)
at org.mapdb.SerializerPojo.serialize(SerializerPojo.java:696)
at org.mapdb.HTreeMap$2.serialize(HTreeMap.java:129)
at org.mapdb.HTreeMap$2.serialize(HTreeMap.java:121)
at org.mapdb.Store.serialize(Store.java:154)
at org.mapdb.StoreWAL.update(StoreWAL.java:371)
at org.mapdb.Caches$HashTable.update(Caches.java:270)
at org.mapdb.HTreeMap.putInner(HTreeMap.java:533)
at org.mapdb.HTreeMap.put(HTreeMap.java:488)
at io.moquette.spi.persistence.MapDBSessionsStore.inFlight(MapDBSessionsStore.java:210)
at io.moquette.spi.ClientSession.inFlightAckWaiting(ClientSession.java:180)
at io.moquette.spi.impl.ProtocolProcessor.route2Subscribers(ProtocolProcessor.java:554)
at io.moquette.spi.impl.ProtocolProcessor.processPublish(ProtocolProcessor.java:416)
at io.moquette.server.netty.NettyMQTTHandler.channelRead(NettyMQTTHandler.java:61)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:304)
at io.moquette.server.netty.metrics.MessageMetricsHandler.channelRead(MessageMetricsHandler.java:46)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:304)
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:276)
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:263)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:304)
at io.netty.channel.ChannelInboundHandlerAdapter.channelRead(ChannelInboundHandlerAdapter.java:86)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:304)
at io.netty.channel.ChannelInboundHandlerAdapter.channelRead(ChannelInboundHandlerAdapter.java:86)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:304)
at io.moquette.server.netty.metrics.BytesMetricsHandler.channelRead(BytesMetricsHandler.java:47)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:304)
at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:266)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:318)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:304)
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:846)
at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:131)
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:511)
at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:468)
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:382)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:354)
at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:112)
at java.lang.Thread.run(Thread.java:745)

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