Unexpected error: null when run multiple threads

[namnhatdoan]

I run a Concurrency Thread Group with following samplers: 100 thread ramp up in 100 sec with ramp-up steps count is 10

• WebSocket Sampler (named Open Connection): Select Streaming Connection

• Loop forever

  • WebSocket Sampler (named Monitor): Select Streaming Connection

After running, I got following error:

[Execution Flow]

• Opening new connection
• Using response message pattern ""
• Using disconnect pattern ""
• Waiting for the server connection for 20000 MILLISECONDS
• WebSocket conection has been opened
• Connection established
• Waiting for messages for 20000 MILLISECONDS
• Received frame Installing plugin #1 (23 bytes); matched response pattern
• Received message Hi, does the websocket sampler need to be configured with the webserver and port details? #2 (23 bytes) - Leaving streaming connection open
  ; matched response pattern

[Variables]

• Message count: 2

[Problems]

• Unexpected error: null
  java.util.LinkedList$ListItr.checkForComodification(Unknown Source)
  java.util.LinkedList$ListItr.next(Unknown Source)
  JMeter.plugins.functional.samplers.websocket.ServiceSocket.getResponseMessage(ServiceSocket.java:149)
  JMeter.plugins.functional.samplers.websocket.WebSocketSampler.sample(WebSocketSampler.java:159)
  org.apache.jmeter.threads.JMeterThread.doSampling(JMeterThread.java:622)
  org.apache.jmeter.threads.JMeterThread.executeSamplePackage(JMeterThread.java:546)
  org.apache.jmeter.threads.JMeterThread.processSampler(JMeterThread.java:486)
  org.apache.jmeter.threads.JMeterThread.run(JMeterThread.java:253)
  java.lang.Thread.run(Unknown Source)

Can anyone help? I don't really understand what this issue means and how it happens.