To reproduce, just start mininet and see first mininet connection open/close generates NPE:

2017-09-26 20:16:41,722 | INFO | entLoopGroup-7-1 | SystemNotificationsListenerImpl | 193 - org.opendaylight.openflowplugin.impl - 0.4.2.SNAPSHOT | ConnectionEvent: Connection closed by device, Device:/10.29.14.171:58518, NodeId:null
2017-09-26 20:16:41,726 | WARN | entLoopGroup-7-1 | DefaultChannelPipeline | 111 - io.netty.common - 4.1.8.Final | An exceptionCaught() event was fired, and it reached at the tail of the pipeline. It usually means the last handler in the pipeline did not handle the exception.
java.lang.NullPointerException
at java.util.concurrent.ConcurrentHashMap.get(ConcurrentHashMap.java:936)[:1.8.0_141]
at org.opendaylight.openflowplugin.impl.lifecycle.ContextChainHolderImpl.onDeviceDisconnected(ContextChainHolderImpl.java:203)[193:org.opendaylight.openflowplugin.impl:0.4.2.SNAPSHOT]
at org.opendaylight.openflowplugin.impl.connection.ConnectionContextImpl.propagateDeviceDisconnectedEvent(ConnectionContextImpl.java:179)[193:org.opendaylight.openflowplugin.impl:0.4.2.SNAPSHOT]
at org.opendaylight.openflowplugin.impl.connection.ConnectionContextImpl.disconnectDevice(ConnectionContextImpl.java:168)[193:org.opendaylight.openflowplugin.impl:0.4.2.SNAPSHOT]
at org.opendaylight.openflowplugin.impl.connection.ConnectionContextImpl.onConnectionClosed(ConnectionContextImpl.java:129)[193:org.opendaylight.openflowplugin.impl:0.4.2.SNAPSHOT]
at org.opendaylight.openflowplugin.impl.connection.listener.SystemNotificationsListenerImpl.onDisconnectEvent(SystemNotificationsListenerImpl.java:57)[193:org.opendaylight.openflowplugin.impl:0.4.2.SNAPSHOT]
at org.opendaylight.openflowjava.protocol.impl.core.connection.ConnectionAdapterImpl.consumeDeviceMessage(ConnectionAdapterImpl.java:100)[182:org.opendaylight.openflowjava.openflow-protocol-impl:0.9.2.SNAPSHOT]
at org.opendaylight.openflowjava.protocol.impl.core.connection.AbstractConnectionAdapterStatistics.consume(AbstractConnectionAdapterStatistics.java:66)[182:org.opendaylight.openflowjava.openflow-protocol-impl:0.9.2.SNAPSHOT]
at org.opendaylight.openflowjava.protocol.impl.core.connection.ConnectionAdapterImpl.consume(ConnectionAdapterImpl.java:44)[182:org.opendaylight.openflowjava.openflow-protocol-impl:0.9.2.SNAPSHOT]
at org.opendaylight.openflowjava.protocol.impl.core.DelegatingInboundHandler.channelInactive(DelegatingInboundHandler.java:52)[182:org.opendaylight.openflowjava.openflow-protocol-impl:0.9.2.SNAPSHOT]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:246)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:232)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:225)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.ChannelInboundHandlerAdapter.channelInactive(ChannelInboundHandlerAdapter.java:75)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:246)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:232)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:225)[112:io.netty.transport:4.1.8.Final]
at io.netty.handler.codec.ByteToMessageDecoder.channelInputClosed(ByteToMessageDecoder.java:360)[114:io.netty.codec:4.1.8.Final]
at io.netty.handler.codec.ByteToMessageDecoder.channelInactive(ByteToMessageDecoder.java:325)[114:io.netty.codec:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:246)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:232)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:225)[112:io.netty.transport:4.1.8.Final]
at io.netty.handler.codec.ByteToMessageDecoder.channelInputClosed(ByteToMessageDecoder.java:360)[114:io.netty.codec:4.1.8.Final]
at io.netty.handler.codec.ByteToMessageDecoder.channelInactive(ByteToMessageDecoder.java:325)[114:io.netty.codec:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:246)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:232)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:225)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.ChannelInboundHandlerAdapter.channelInactive(ChannelInboundHandlerAdapter.java:75)[112:io.netty.transport:4.1.8.Final]
at io.netty.handler.timeout.IdleStateHandler.channelInactive(IdleStateHandler.java:278)[116:io.netty.handler:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:246)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:232)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:225)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.ChannelInboundHandlerAdapter.channelInactive(ChannelInboundHandlerAdapter.java:75)[112:io.netty.transport:4.1.8.Final]
at org.opendaylight.openflowjava.protocol.impl.core.connection.ChannelOutboundQueue.channelInactive(ChannelOutboundQueue.java:256)[182:org.opendaylight.openflowjava.openflow-protocol-impl:0.9.2.SNAPSHOT]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:246)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:232)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelInactive(AbstractChannelHandlerContext.java:225)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelInactive(DefaultChannelPipeline.java:1329)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:246)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelInactive(AbstractChannelHandlerContext.java:232)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.DefaultChannelPipeline.fireChannelInactive(DefaultChannelPipeline.java:908)[112:io.netty.transport:4.1.8.Final]
at io.netty.channel.AbstractChannel$AbstractUnsafe$7.run(AbstractChannel.java:744)[112:io.netty.transport:4.1.8.Final]
at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:163)[111:io.netty.common:4.1.8.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasksFrom(SingleThreadEventExecutor.java:379)[111:io.netty.common:4.1.8.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:354)[111:io.netty.common:4.1.8.Final]
at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:292)[117:io.netty.transport-native-epoll:4.1.8.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858)[111:io.netty.common:4.1.8.Final]
at io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:144)[111:io.netty.common:4.1.8.Final]
at java.lang.Thread.run(Thread.java:748)[:1.8.0_141]
2017-09-26 20:16:41,729 | INFO | entLoopGroup-7-1 | SystemNotificationsListenerImpl | 193 - org.opendaylight.openflowplugin.impl - 0.4.2.SNAPSHOT | ConnectionEvent: Connection closed by device, Device:/10.29.14.171:58518, NodeId:null

This issue is impacting SW scalability test as we are looking for NPEs and stop the test when find them.