Logstash "Enable Globally" causes never ending stage log loops

As commented on https://github.com/jenkinsci/logstash-plugin/pull/93

Since we picked up 2.4.0 last week we've had some odd symptoms such as console logs for stages looping indefinitely in the master, and then spotted this stack in the slave image logs;

{{WARNING: null
java.lang.IllegalStateException: Jenkins.instance is missing. Read the documentation of Jenkins.getInstanceOrNull to see what you are doing wrong.
at jenkins.model.Jenkins.get(Jenkins.java:777)
at jenkins.model.GlobalConfiguration.all(GlobalConfiguration.java:75)
at jenkins.plugins.logstash.LogstashConfiguration.getInstance(LogstashConfiguration.java:289)
at jenkins.plugins.logstash.LogstashWriter.getIndexerDao(LogstashWriter.java:162)
at jenkins.plugins.logstash.LogstashWriter.getDaoOrNull(LogstashWriter.java:201)
at jenkins.plugins.logstash.LogstashWriter.<init>(LogstashWriter.java:82)
at jenkins.plugins.logstash.pipeline.GlobalDecorator.decorate(GlobalDecorator.java:41)
at org.jenkinsci.plugins.workflow.log.TaskListenerDecorator$DecoratedTaskListener.getLogger(TaskListenerDecorator.java:237)
at org.jenkinsci.plugins.workflow.log.TaskListenerDecorator$CloseableTaskListener.getLogger(TaskListenerDecorator.java:279)
at org.jenkinsci.plugins.gitclient.CliGitAPIImpl$2.execute(CliGitAPIImpl.java:740)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler$GitCommandMasterToSlaveCallable.call(RemoteGitImpl.java:161)
at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler$GitCommandMasterToSlaveCallable.call(RemoteGitImpl.java:154)
at hudson.remoting.UserRequest.perform(UserRequest.java:212)
at hudson.remoting.UserRequest.perform(UserRequest.java:54)
at hudson.remoting.Request$2.run(Request.java:369)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at hudson.remoting.Engine$1.lambda$newThread$0(Engine.java:93)
at java.lang.Thread.run(Thread.java:748)}}

If we turn off "Enable Globally" all is well again (except no logstash output of course )