Detecting why this exception happens:

mavenExecutionResult exceptions not empty
message : Internal error: java.lang.NullPointerException
cause : null
Stack trace :
org.apache.maven.InternalErrorException: Internal error: java.lang.NullPointerException
	at org.apache.maven.lifecycle.internal.BuilderCommon.handleBuildError(BuilderCommon.java:128)
	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:95)
	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:59)
	at org.apache.maven.lifecycle.internal.LifecycleStarter.singleThreadedBuild(LifecycleStarter.java:183)
	at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:161)
	at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:319)
	at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:156)
	at org.jvnet.hudson.maven3.launcher.Maven3Launcher.main(Maven3Launcher.java:79)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launchStandard(Launcher.java:329)
	at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:239)
	at org.jvnet.hudson.maven3.agent.Maven3Main.launch(Maven3Main.java:158)
	at hudson.maven.Maven3Builder.call(Maven3Builder.java:121)
	at hudson.maven.Maven3Builder.call(Maven3Builder.java:73)
	at hudson.remoting.UserRequest.perform(UserRequest.java:118)
	at hudson.remoting.UserRequest.perform(UserRequest.java:48)
	at hudson.remoting.Request$2.run(Request.java:287)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
	at java.util.concurrent.FutureTask.run(FutureTask.java:138)
	at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
	at java.lang.Thread.run(Thread.java:662)
Caused by: java.lang.NullPointerException
	at hudson.maven.MojoInfo.intercept(MojoInfo.java:208)
	at hudson.maven.reporters.MavenArtifactArchiver.preExecute(MavenArtifactArchiver.java:74)
	at hudson.maven.Maven3Builder$MavenExecutionListener.recordMojoStarted(Maven3Builder.java:535)
	at hudson.maven.Maven3Builder$MavenExecutionListener.forkStarted(Maven3Builder.java:671)
	at org.apache.maven.lifecycle.internal.DefaultExecutionEventCatapult.fire(DefaultExecutionEventCatapult.java:94)
	at org.apache.maven.lifecycle.internal.DefaultExecutionEventCatapult.fire(DefaultExecutionEventCatapult.java:42)
	at org.apache.maven.lifecycle.internal.MojoExecutor.executeForkedExecutions(MojoExecutor.java:328)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:199)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
	at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
	at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:84)
	... 24 more
channel stopped
FATAL: null
java.lang.NullPointerException
	at hudson.maven.MavenModuleSetBuild$RunnerImpl.doRun(MavenModuleSetBuild.java:833)
	at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:448)
	at hudson.model.Run.run(Run.java:1376)
	at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:479)
	at hudson.model.ResourceController.execute(ResourceController.java:88)
	at hudson.model.Executor.run(Executor.java:230)

maven-plugin/src/main/java/hudson/maven/MojoInfo.java

@@ -92,6 +92,8 @@ public class MojoInfo {
     private final ConverterLookup converterLookup = new DefaultConverterLookup();
 
     public MojoInfo(MojoExecution mojoExecution, Mojo mojo, PlexusConfiguration configuration, ExpressionEvaluator expressionEvaluator) {
+        if (mojo==null)     throw new IllegalArgumentException();
+        // TODO: exactly what other variables are always non-null?
         this.mojo = mojo;
         this.mojoExecution = mojoExecution;
         this.configuration = configuration;
@@ -130,14 +132,13 @@ public class MojoInfo {
         PlexusConfiguration child = configuration.getChild(configName);
         if(child==null) return null;    // no such config
        
-        ClassLoader cl = null;
+        ClassLoader cl;
         PluginDescriptor pd = mojoExecution.getMojoDescriptor().getPluginDescriptor();
         // for maven2 builds ClassRealm doesn't extends ClassLoader !
         // so check stuff with reflection
         Method method = ReflectionUtils.getPublicMethodNamed( pd.getClass(), "getClassRealm" );
        
-        if ( ReflectionUtils.invokeMethod( method, pd ) instanceof ClassRealm)
-        {
+        if ( ReflectionUtils.invokeMethod( method, pd ) instanceof ClassRealm) {
             ClassRealm cr = (ClassRealm) ReflectionUtils.invokeMethod( method, pd );
             cl = cr.getClassLoader();
         } else {