I found the problem. I tried to debug the code to see what's going on and well there's a line of code which doesn't pass:
PackageRegistry pkgRegistry = this.packageBuilder.getPackageRegistry(process.getPackageName());
and if pkgRegistry is null it throws this exception... In my case it is null because in packageRegistry there's only defaultPackage package and process.getPackageName() returns org.drools.bpmn2 and I don;t know why it's filled with this name... Any assumptions?