Nullpointer in SonargraphSensor if a maven parent project contains modules that have no sources

I have a maven parent project that consists of several sub modules. One of these projects uses a custom package type and does not have any sources. If this module is included in the build then I get the following error

[ERROR] Failed to execute goal org.codehaus.mojo:sonar-maven-plugin:2.0:sonar (default-cli) on project cbdp-maven-playground: Can not execute Sonar: NullPointerException -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.codehaus.mojo:sonar-maven-plugin:2.0:sonar (default-cli) on project cbdp-maven-playground: Can not execute Sonar
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:217)
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)
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.apache.maven.cli.MavenCli.execute(MavenCli.java:537)
at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:196)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:141)
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.launchEnhanced(Launcher.java:290)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:230)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:409)
at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:352)
Caused by: org.apache.maven.plugin.MojoExecutionException: Can not execute Sonar
at org.codehaus.mojo.sonar.Bootstraper.executeMojo(Bootstraper.java:118)
at org.codehaus.mojo.sonar.Bootstraper.start(Bootstraper.java:65)
at org.codehaus.mojo.sonar.SonarMojo.execute(SonarMojo.java:90)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:101)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:209)
... 19 more
Caused by: java.lang.NullPointerException
at com.hello2morrow.sonarplugin.api.SonargraphSensor.analyseCylceGroups(SonargraphSensor.java:270)
at com.hello2morrow.sonarplugin.api.SonargraphSensor.analyse(SonargraphSensor.java:154)
at org.sonar.batch.phases.SensorsExecutor.execute(SensorsExecutor.java:64)
at org.sonar.batch.phases.Phases.execute(Phases.java:93)
at org.sonar.batch.bootstrap.ProjectModule.doStart(ProjectModule.java:143)
at org.sonar.batch.bootstrap.Module.start(Module.java:83)
at org.sonar.batch.bootstrap.BatchModule.analyze(BatchModule.java:115)
at org.sonar.batch.bootstrap.BatchModule.analyze(BatchModule.java:110)
at org.sonar.batch.bootstrap.BatchModule.doStart(BatchModule.java:105)
at org.sonar.batch.bootstrap.Module.start(Module.java:83)
at org.sonar.batch.bootstrap.BootstrapModule.doStart(BootstrapModule.java:109)
at org.sonar.batch.bootstrap.Module.start(Module.java:83)
at org.sonar.batch.bootstrapper.Batch.startBatch(Batch.java:72)
at org.sonar.batch.bootstrapper.Batch.execute(Batch.java:59)
at org.sonar.maven3.SonarMojo.execute(SonarMojo.java:143)
at org.codehaus.mojo.sonar.Bootstraper.executeMojo(Bootstraper.java:113)
... 23 more