prerequisite:
at least one running maven job

1. open Manage Jenkins - Configure System
2. click add maven
3. click delete installer
4. move the (empty) maven to the top of the list of maven installations
5. click save
6. run the maven job

expected: maven job runs as before

result:
ERROR: Processing failed due to a bug in the code. Please report this to jenkinsci-users@googlegroups.com
java.lang.NullPointerException
at hudson.maven.MavenModuleSet.getMaven(MavenModuleSet.java:982)
at hudson.maven.MavenModuleSetBuild.getEnvironment(MavenModuleSetBuild.java:160)
at hudson.maven.MavenModuleSetBuild$MavenModuleSetBuildExecution.doRun(MavenModuleSetBuild.java:568)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:499)
at hudson.model.Run.execute(Run.java:1484)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:477)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:236)
project=hudson.maven.MavenModuleSet@16c8cb41
[... some list of modules here ... ]

FATAL: null
java.lang.NullPointerException
at hudson.maven.MavenModuleSet.getMaven(MavenModuleSet.java:982)
at hudson.maven.MavenModuleSetBuild.getEnvironment(MavenModuleSetBuild.java:160)
at hudson.maven.MavenModuleSetBuild$MavenModuleSetBuildExecution.doRun(MavenModuleSetBuild.java:568)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:499)
at hudson.model.Run.execute(Run.java:1484)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:477)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:236)

my hudson.tasks.Maven.xml looks like this:
<?xml version='1.0' encoding='UTF-8'?>
<hudson.tasks.Maven_-DescriptorImpl>
<helpRedirect/>
<installations>
<hudson.tasks.Maven_-MavenInstallation>
<properties>
<hudson.tools.InstallSourceProperty>
<installers/>
</hudson.tools.InstallSourceProperty>
</properties>
</hudson.tasks.Maven_-MavenInstallation>
<hudson.tasks.Maven_-MavenInstallation>
<name>/cvsroot/maven/maven-bin/maven-2.0.11 (depreciated - for Java 1.4 only)</name>
<home>/var/hudson/tools/maven-bin/maven-2.0.11</home>
<properties/>
</hudson.tasks.Maven_-MavenInstallation>
<hudson.tasks.Maven_-MavenInstallation>
<name>/cvsroot/maven/maven-bin/maven-2.2.1 (Java 1.5+ required)</name>
<home>/var/hudson/tools/maven-bin/maven-2.2.1</home>
<properties/>
</hudson.tasks.Maven_-MavenInstallation>
<hudson.tasks.Maven_-MavenInstallation>
<name>/cvsroot/maven/maven-bin/maven-3.0.x</name>
<home>/var/hudson/tools/maven-bin/maven-3.0.x</home>
<properties/>
</hudson.tasks.Maven_-MavenInstallation>
</installations>
</hudson.tasks.Maven_-DescriptorImpl>

the fix is simple: just delete the empty maven configuration