#vulnerable code 
                             private ParallelExecutorMapping mapMethods(ClassGroup one, ClassGroup two)
	{
		MethodSignatureMapper msm = new MethodSignatureMapper();
		msm.map(one, two);

		List<ParallelExecutorMapping> pmes = new ArrayList<>();

		for (Method m : msm.getMap().keySet())
		{
			Collection<Method> methods = msm.getMap().get(m);

			ExecutionMapper em = new ExecutionMapper(m, methods);

			ParallelExecutorMapping mapping = em.run();
			mapping.map(mapping.m1, mapping.m2);

			pmes.add(mapping);
		}

		ParallelExecutorMapping finalm = new ParallelExecutorMapping(one, two);
		for (ParallelExecutorMapping pme : pmes)
			finalm.merge(pme);

		return finalm;
	} 
                            #location 15 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                             private ParallelExecutorMapping mapMethods(ClassGroup one, ClassGroup two)
	{
		MethodSignatureMapper msm = new MethodSignatureMapper();
		msm.map(one, two);

		List<ParallelExecutorMapping> pmes = new ArrayList<>();

		for (Method m : msm.getMap().keySet())
		{
			Collection<Method> methods = msm.getMap().get(m);

			ExecutionMapper em = new ExecutionMapper(m, methods);

			ParallelExecutorMapping mapping = em.run();
			if (mapping == null)
				continue;

			mapping.map(mapping.m1, mapping.m2);

			pmes.add(mapping);
		}

		ParallelExecutorMapping finalm = new ParallelExecutorMapping(one, two);
		for (ParallelExecutorMapping pme : pmes)
			finalm.merge(pme);

		return finalm;
	}