Message: invalid argument: can't kill an exited process

[BeyondEvil]

Full stacktrace:

E   selenium.common.exceptions.WebDriverException: Message: invalid argument: can't kill an exited process
E   Build info: version: '3.14.0', revision: 'aacccce0', time: '2018-08-02T20:13:22.693Z'
E   System info: host: 'bd234cc79e62', ip: '172.17.0.2', os.name: 'Linux', os.arch: 'amd64', os.version: '4.9.125-linuxkit', java.version: '1.8.0_181'
E   Driver info: driver.version: unknown
E   remote stacktrace: 
E   Stacktrace:
E       at sun.reflect.NativeConstructorAccessorImpl.newInstance0 (NativeConstructorAccessorImpl.java:-2)
E       at sun.reflect.NativeConstructorAccessorImpl.newInstance (NativeConstructorAccessorImpl.java:62)
E       at sun.reflect.DelegatingConstructorAccessorImpl.newInstance (DelegatingConstructorAccessorImpl.java:45)
E       at java.lang.reflect.Constructor.newInstance (Constructor.java:423)
E       at org.openqa.selenium.remote.W3CHandshakeResponse.lambda$new$0 (W3CHandshakeResponse.java:57)
E       at org.openqa.selenium.remote.W3CHandshakeResponse.lambda$getResponseFunction$2 (W3CHandshakeResponse.java:104)
E       at org.openqa.selenium.remote.ProtocolHandshake.lambda$createSession$0 (ProtocolHandshake.java:122)
E       at java.util.stream.ReferencePipeline$3$1.accept (ReferencePipeline.java:193)
E       at java.util.Spliterators$ArraySpliterator.tryAdvance (Spliterators.java:958)
E       at java.util.stream.ReferencePipeline.forEachWithCancel (ReferencePipeline.java:126)
E       at java.util.stream.AbstractPipeline.copyIntoWithCancel (AbstractPipeline.java:498)
E       at java.util.stream.AbstractPipeline.copyInto (AbstractPipeline.java:485)
E       at java.util.stream.AbstractPipeline.wrapAndCopyInto (AbstractPipeline.java:471)
E       at java.util.stream.FindOps$FindOp.evaluateSequential (FindOps.java:152)
E       at java.util.stream.AbstractPipeline.evaluate (AbstractPipeline.java:234)
E       at java.util.stream.ReferencePipeline.findFirst (ReferencePipeline.java:464)
E       at org.openqa.selenium.remote.ProtocolHandshake.createSession (ProtocolHandshake.java:125)
E       at org.openqa.selenium.remote.ProtocolHandshake.createSession (ProtocolHandshake.java:73)
E       at org.openqa.selenium.remote.server.RemoteSession$Factory.performHandshake (RemoteSession.java:145)
E       at org.openqa.selenium.remote.server.ServicedSession$Factory.apply (ServicedSession.java:160)
E       at org.openqa.selenium.remote.server.ActiveSessionFactory.lambda$apply$12 (ActiveSessionFactory.java:177)
E       at java.util.stream.ReferencePipeline$3$1.accept (ReferencePipeline.java:193)
E       at java.util.stream.ReferencePipeline$11$1.accept (ReferencePipeline.java:373)
E       at java.util.stream.ReferencePipeline$2$1.accept (ReferencePipeline.java:175)
E       at java.util.Spliterators$ArraySpliterator.tryAdvance (Spliterators.java:958)
E       at java.util.stream.ReferencePipeline.forEachWithCancel (ReferencePipeline.java:126)
E       at java.util.stream.AbstractPipeline.copyIntoWithCancel (AbstractPipeline.java:498)
E       at java.util.stream.AbstractPipeline.copyInto (AbstractPipeline.java:485)
E       at java.util.stream.AbstractPipeline.wrapAndCopyInto (AbstractPipeline.java:471)
E       at java.util.stream.FindOps$FindOp.evaluateSequential (FindOps.java:152)
E       at java.util.stream.AbstractPipeline.evaluate (AbstractPipeline.java:234)
E       at java.util.stream.ReferencePipeline.findFirst (ReferencePipeline.java:464)
E       at org.openqa.selenium.remote.server.ActiveSessionFactory.apply (ActiveSessionFactory.java:180)
E       at org.openqa.selenium.remote.server.NewSessionPipeline.lambda$null$2 (NewSessionPipeline.java:65)
E       at java.util.stream.ReferencePipeline$3$1.accept (ReferencePipeline.java:193)
E       at java.util.stream.ReferencePipeline$2$1.accept (ReferencePipeline.java:175)
E       at java.util.Collections$2.tryAdvance (Collections.java:4717)
E       at java.util.stream.ReferencePipeline.forEachWithCancel (ReferencePipeline.java:126)
E       at java.util.stream.AbstractPipeline.copyIntoWithCancel (AbstractPipeline.java:498)
E       at java.util.stream.AbstractPipeline.copyInto (AbstractPipeline.java:485)
E       at java.util.stream.AbstractPipeline.wrapAndCopyInto (AbstractPipeline.java:471)
E       at java.util.stream.FindOps$FindOp.evaluateSequential (FindOps.java:152)
E       at java.util.stream.AbstractPipeline.evaluate (AbstractPipeline.java:234)
E       at java.util.stream.ReferencePipeline.findFirst (ReferencePipeline.java:464)
E       at org.openqa.selenium.remote.server.NewSessionPipeline.lambda$createNewSession$3 (NewSessionPipeline.java:68)
E       at java.util.stream.ReferencePipeline$3$1.accept (ReferencePipeline.java:193)
E       at java.util.stream.ReferencePipeline$3$1.accept (ReferencePipeline.java:193)
E       at java.util.stream.ReferencePipeline$3$1.accept (ReferencePipeline.java:193)
E       at java.util.stream.DistinctOps$1$2.accept (DistinctOps.java:175)
E       at java.util.stream.ReferencePipeline$2$1.accept (ReferencePipeline.java:175)
E       at java.util.stream.ReferencePipeline$3$1.accept (ReferencePipeline.java:193)
E       at java.util.stream.ReferencePipeline$2$1.accept (ReferencePipeline.java:175)
E       at java.util.stream.Streams$StreamBuilderImpl.tryAdvance (Streams.java:405)
E       at java.util.stream.Streams$ConcatSpliterator.tryAdvance (Streams.java:728)
E       at java.util.stream.ReferencePipeline.forEachWithCancel (ReferencePipeline.java:126)
E       at java.util.stream.AbstractPipeline.copyIntoWithCancel (AbstractPipeline.java:498)
E       at java.util.stream.AbstractPipeline.copyInto (AbstractPipeline.java:485)
E       at java.util.stream.AbstractPipeline.wrapAndCopyInto (AbstractPipeline.java:471)
E       at java.util.stream.FindOps$FindOp.evaluateSequential (FindOps.java:152)
E       at java.util.stream.AbstractPipeline.evaluate (AbstractPipeline.java:234)
E       at java.util.stream.ReferencePipeline.findFirst (ReferencePipeline.java:464)
E       at org.openqa.selenium.remote.server.NewSessionPipeline.createNewSession (NewSessionPipeline.java:71)
E       at org.openqa.selenium.remote.server.commandhandler.BeginSession.execute (BeginSession.java:65)
E       at org.openqa.selenium.remote.server.WebDriverServlet.lambda$handle$3 (WebDriverServlet.java:250)
E       at java.util.concurrent.Executors$RunnableAdapter.call (Executors.java:511)
E       at java.util.concurrent.FutureTask.run (FutureTask.java:266)
E       at java.util.concurrent.ThreadPoolExecutor.runWorker (ThreadPoolExecutor.java:1149)
E       at java.util.concurrent.ThreadPoolExecutor$Worker.run (ThreadPoolExecutor.java:624)
E       at java.lang.Thread.run (Thread.java:748)

This is the result of having set START_XVFB environment variable to false, and then forgot to run Firefox in headless mode.