#vulnerable code 
                             @Override
	public List<Feature> encodeFeatures(List<String> ids, List<Feature> features, SkLearnEncoder encoder){
		List<?> data = getData();

		if(ids.size() != 1 || features.size() != 1){
			throw new IllegalArgumentException();
		}

		final
		InvalidValueTreatmentMethod invalidValueTreatment = DomainUtil.parseInvalidValueTreatment(getInvalidValueTreatment());

		WildcardFeature wildcardFeature = (WildcardFeature)features.get(0);

		Function<Object, String> function = new Function<Object, String>(){

			@Override
			public String apply(Object object){
				return ValueUtil.formatValue(object);
			}
		};

		List<String> categories = Lists.transform(data, function);

		FieldDecorator decorator = new ValidValueDecorator(){

			{
				setInvalidValueTreatment(invalidValueTreatment);
			}
		};

		CategoricalFeature categoricalFeature = wildcardFeature.toCategoricalFeature(categories);

		encoder.addDecorator(categoricalFeature.getName(), decorator);

		return Collections.<Feature>singletonList(categoricalFeature);
	} 
                            #location 10 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                             @Override
	public List<Feature> encodeFeatures(List<String> ids, List<Feature> features, SkLearnEncoder encoder){
		List<?> data = getData();

		ClassDictUtil.checkSize(1, ids, features);

		final
		InvalidValueTreatmentMethod invalidValueTreatment = DomainUtil.parseInvalidValueTreatment(getInvalidValueTreatment());

		WildcardFeature wildcardFeature = (WildcardFeature)features.get(0);

		Function<Object, String> function = new Function<Object, String>(){

			@Override
			public String apply(Object object){
				return ValueUtil.formatValue(object);
			}
		};

		List<String> categories = Lists.transform(data, function);

		FieldDecorator decorator = new ValidValueDecorator(){

			{
				setInvalidValueTreatment(invalidValueTreatment);
			}
		};

		CategoricalFeature categoricalFeature = wildcardFeature.toCategoricalFeature(categories);

		encoder.addDecorator(categoricalFeature.getName(), decorator);

		return Collections.<Feature>singletonList(categoricalFeature);
	}