I think the issue is similar to

https://issues.jenkins-ci.org/browse/JENKINS-29249

I got error log. The issue is seldom. It should be happen after run a long time :

15:06:41 Notifying upstream projects of job completion*15:06:41* Email was triggered for: Always*15:06:41* Sending email for trigger: Always*15:06:41* ERROR: Could not send email as a part of the post-build publishers.15:06:41 java.lang.NullPointerException15:06:41       at hudson.plugins.emailext.plugins.recipients.RequesterRecipientProvider.addRecipients(RequesterRecipientProvider.java:56)15:06:41       at hudson.plugins.emailext.ExtendedEmailPublisher.createMail(ExtendedEmailPublisher.java:637)15:06:41       at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:355)15:06:41       at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:345)15:06:41       at hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:253)15:06:41       at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)15:06:41       at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:779)15:06:41       at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:720)15:06:41       at hudson.model.Build$BuildExecution.cleanUp(Build.java:195)15:06:41       at hudson.model.Run.execute(Run.java:1775)15:06:41       at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)15:06:41       at hudson.model.ResourceController.execute(ResourceController.java:98)15:06:41       at hudson.model.Executor.run(Executor.java:404)

It is a daily build job.

The job contains several trigger jobs.

And 3 Post-build Actions:

1. log parser
2. send mail
3. trigger daily_build_monitor

It is a recursive trigger. Objectivity is for monitor daily build generate.

14:02:36 Started by upstream project "daily_build_monitor" build number 579
14:02:36 originally caused by:
14:02:36 Started by upstream project "daily_build_monitor" build number 578
14:02:36 originally caused by:
14:02:36 Started by upstream project "daily_build_monitor" build number 577
14:02:36 originally caused by:
14:02:36 Started by upstream project "daily_build_monitor" build number 576
14:02:36 originally caused by:
14:02:36 Started by upstream project "daily_build_monitor" build number 575
14:02:36 originally caused by:
14:02:36 Started by upstream project "daily_build_monitor" build number 574
14:02:36 originally caused by:
14:02:36 Started by upstream project "daily_build_monitor" build number 573
14:02:36 originally caused by:
14:02:36 Started by upstream project "daily_build_monitor" build number 572
14:02:36 originally caused by:
14:02:36 Started by upstream project "daily_build_monitor" build number 571
14:02:36 originally caused by:
14:02:36 Started by upstream project "daily_build_monitor" build number 570
14:02:36 originally caused by:
14:02:36 Started by upstream project "daily_build_monitor" build number 569
14:02:36 originally caused by:
14:02:36 Started by upstream project "daily_build_monitor" build number 568
14:02:36 originally caused by:
14:02:36 (deeply nested causes)

And this issue similar following link for you to refer

https://issues.jenkins-ci.org/browse/JENKINS-23269

Suggestion:
either return on "build == null"
or check Run upstream = job.getBuildByNumber(upstreamCause.getUpstreamBuild()); for null too