java.lang.IllegalArgumentException: Null value not allowed as an environment variable: M2_HOME 
	at hudson.EnvVars.put(EnvVars.java:172) 
	at hudson.tasks.Maven$MavenInstallation.buildEnvVars(Maven.java:489) 
	at hudson.maven.MavenModuleSetBuild.getEnvironment(MavenModuleSetBuild.java:172) 
	at hudson.scm.SubversionSCM.compareRemoteRevisionWith(SubversionSCM.java:1230) 
	at hudson.scm.SCM._compareRemoteRevisionWith(SCM.java:356) 
	at hudson.scm.SCM.poll(SCM.java:373) 
	at hudson.model.AbstractProject._poll(AbstractProject.java:1521) 
	at hudson.model.AbstractProject.poll(AbstractProject.java:1446) 
	at hudson.triggers.SCMTrigger$Runner.runPolling(SCMTrigger.java:439) 
	at hudson.triggers.SCMTrigger$Runner.run(SCMTrigger.java:468) 
	at hudson.util.SequentialExecutionQueue$QueueEntry.run(SequentialExecutionQueue.java:118) 
	at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) 
	at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source) 
	at java.util.concurrent.FutureTask.run(Unknown Source) 
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) 
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) 
	at java.lang.Thread.run(Unknown Source)