NPE when calling generate goal

[BuBuaBu]

When a call the generate goal on a multi module project, I get a NPE.

project structure to reproduce:
foo/
|---pom.xml
|---bar/
    |---pom.xml
|---baz/
    |---pom.xml

foo/pom.xml do not call the plugin.
The plugin is called only on foob/bar/pom.xml

Here is the stack trace: https://gist.github.com/BuBuaBu/e107a7b8af72070e56fe

[BuBuaBu]

I'm not sure that this king of usage is allowed.
In the situation we are trying to create an incomplete report as foo/pom.xml will not be part of the report making foo/bar/pom.xml orphan.

If so I'm expecting to get a better error rather an NPE.

In org.axway.grapes.maven.report.ModuleAggregator.updateParent(MavenProject, Module), we are retrieving the parent report, but this one does not exit, thus parentModuleFile.exists() is false and GrapesMavenPlugin.getModule(workingFolder, GrapesMavenPlugin.MODULE_JSON_FILE_NAME) return null.
I add this line to workaround the issue:
if (rootModule == null){ return false;}

But way may throw an error instead.

What do you think?