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java.lang.NullPointerException: null	at java.util.concurrent.ConcurrentHashMap.hash(ConcurrentHashMap.java:333)	at java.util.concurrent.ConcurrentHashMap.remove(ConcurrentHashMap.java:1175)	at org.geppetto.frontend.controllers.ConnectionsManager.removeConnection(ConnectionsManager.java:96)	at org.geppetto.frontend.controllers.ConnectionsManager.purgeLostConnections(ConnectionsManager.java:82)	at org.geppetto.frontend.controllers.ConnectionsManager.addConnection(ConnectionsManager.java:54)	at org.geppetto.frontend.controllers.WebsocketConnection.onOpen(WebsocketConnection.java:122)	at org.apache.catalina.websocket.StreamInbound.onUpgradeComplete(StreamInbound.java:240)	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:603)	at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)