#vulnerable code 
                             @Override
	public List<Feature> encodeFeatures(List<Feature> features, SkLearnEncoder encoder){
		String function = translateFunction(getFunction());

		if(features.size() <= 1){
			return features;
		}

		FieldName name = FieldName.create(function + "(" + FeatureUtil.formatFeatureList(features) + ")");

		Apply apply = new Apply(function);

		for(Feature feature : features){
			apply.addExpressions(feature.ref());
		}

		DerivedField derivedField = encoder.createDerivedField(name, OpType.CONTINUOUS, DataType.DOUBLE, apply);

		return Collections.<Feature>singletonList(new ContinuousFeature(encoder, derivedField));
	} 
                            #location 3 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                             @Override
	public List<Feature> encodeFeatures(List<Feature> features, SkLearnEncoder encoder){
		String function = getFunction();

		if(features.size() <= 1){
			return features;
		}

		Apply apply = new Apply(translateFunction(function));

		for(Feature feature : features){
			apply.addExpressions(feature.ref());
		}

		FieldName name = FieldName.create(function + "(" + FeatureUtil.formatFeatureList(features) + ")"); // XXX

		DerivedField derivedField = encoder.createDerivedField(name, OpType.CONTINUOUS, DataType.DOUBLE, apply);

		return Collections.<Feature>singletonList(new ContinuousFeature(encoder, derivedField));
	}