We use the Groovy-Postbuild-Plugin to set successful Upstream-Builds to failed, when a downstream-Build failes.
The upstream-Build saves several Artefacts and creates Fingerprints from these.
Some times the Groovy-Postbuild-Plugin works with the code below for ten or more builds, afterwards we get the below mentioned NullPointerException several times without changing anything in the configuration.

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Groovy-Code:
upstreamBuilds = manager.build.getUpstreamBuilds();
upstreamJob = upstreamBuilds.keySet().iterator().next();
lastUpstreamBuild = upstreamJob.getLastBuild();
if(lastUpstreamBuild.getResult().isBetterThan(manager.build.result)) {
lastUpstreamBuild.setResult(manager.build.result);
}

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Exception:
Groovy script failed:
java.lang.NullPointerException: Cannot invoke method isBetterThan() on null object
at org.codehaus.groovy.runtime.NullObject.invokeMethod(NullObject.java:77)
at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.call(PogoMetaClassSite.java:45)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:42)
at org.codehaus.groovy.runtime.callsite.NullCallSite.call(NullCallSite.java:32)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:42)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:108)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:116)
at Script1.run(Script1.groovy:23)
at groovy.lang.GroovyShell.evaluate(GroovyShell.java:580)
at groovy.lang.GroovyShell.evaluate(GroovyShell.java:618)
at groovy.lang.GroovyShell.evaluate(GroovyShell.java:589)
at org.jvnet.hudson.plugins.groovypostbuild.GroovyPostbuildRecorder.perform(GroovyPostbuildRecorder.java:273)
at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:717)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:692)
at hudson.model.Build$BuildExecution.post2(Build.java:183)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:639)
at hudson.model.Run.execute(Run.java:1485)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:239)