Intermittent failure in TestRaftStateMachineExceptionWithSimulatedRpc

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Test set: org.apache.ratis.server.simulation.TestRaftStateMachineExceptionWithSimulatedRpc
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Tests run: 3, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 6.239 s <<< FAILURE! - in org.apache.ratis.server.simulation.TestRaftStateMachineExceptionWithSimulatedRpc
org.apache.ratis.server.simulation.TestRaftStateMachineExceptionWithSimulatedRpc.testRetryOnExceptionDuringReplication  Time elapsed: 1.53 s  <<< ERROR!
java.lang.NullPointerException
    at java.base/java.util.Objects.requireNonNull(Objects.java:221)
    at org.apache.ratis.server.impl.RaftStateMachineExceptionTests.runTestRetryOnExceptionDuringReplication(RaftStateMachineExceptionTests.java:172)
    at org.apache.ratis.server.impl.MiniRaftCluster$Factory$Get.runWithNewCluster(MiniRaftCluster.java:141)
    at org.apache.ratis.server.impl.MiniRaftCluster$Factory$Get.runWithNewCluster(MiniRaftCluster.java:121)
    at org.apache.ratis.server.impl.RaftStateMachineExceptionTests.testRetryOnExceptionDuringReplication(RaftStateMachineExceptionTests.java:147)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
    at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:299)
    at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:293)
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    at java.base/java.lang.Thread.run(Thread.java:829)