When a promotion of a build fails (due to staging validation rules) the plugin dumps stack.
[M2Release] Closing repository Stage[profileId=40365b31e0dc2b, stageId=release_staging_profile-418
FATAL: [M2Release] Could not close repository , StageException org.jvnet.hudson.plugins.m2release.nexus.StageException: Failed to perform CLOSE action to nexus stage(Stage[profileId=40365b31e0dc2b, stageId=release_staging_profile-418)

ERROR: Processing failed due to a bug in the code. Please report this to jenkinsci-users@googlegroups.com
java.lang.NullPointerException
at org.jvnet.hudson.plugins.m2release.M2ReleaseBuildWrapper$2.tearDown(M2ReleaseBuildWrapper.java:198)
at hudson.maven.MavenModuleSetBuild$RunnerImpl.doRun(MavenModuleSetBuild.java:754)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:429)
at hudson.model.Run.run(Run.java:1374)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:467)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:145)

THis can be reproduced if you have validation rules on closing a stage (for instance artifact uniqueness where a release version already exists)