#vulnerable code 
                             public void encodeFeatures(SkLearnEncoder encoder){
		List<Object[]> steps = getFeatures();

		for(int row = 0; row < steps.size(); row++){
			Object[] step = steps.get(row);

			List<String> ids = new ArrayList<>();
			List<Feature> features = new ArrayList<>();

			List<String> names = getNameList(step);
			for(String name : names){
				ids.add(name);

				DataField dataField = encoder.createDataField(FieldName.create(name));

				Feature feature = new WildcardFeature(encoder, dataField);

				features.add(feature);
			}

			List<Transformer> transformers = getTransformerList(step);
			for(int column = 0; column < transformers.size(); column++){
				Transformer transformer = transformers.get(column);

				for(Feature feature : features){
					encoder.updateType(feature.getName(), transformer.getOpType(), transformer.getDataType());
				}

				features = transformer.encodeFeatures(ids, features, encoder);
			}

			encoder.addRow(ids, features);
		}
	} 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                             public void encodeFeatures(SkLearnEncoder encoder){
		Object _default = getDefault();
		List<Object[]> rows = getFeatures();

		if(!(Boolean.FALSE).equals(_default)){
			throw new IllegalArgumentException();
		}

		for(Object[] row : rows){
			List<String> ids = new ArrayList<>();
			List<Feature> features = new ArrayList<>();

			List<String> columns = getColumnList(row);
			for(String column : columns){
				FieldName name = FieldName.create(column);

				ids.add(name.getValue());

				DataField dataField = encoder.getDataField(name);
				if(dataField == null){
					dataField = encoder.createDataField(name);
				}

				Feature feature = new WildcardFeature(encoder, dataField);

				features.add(feature);
			}

			List<Transformer> transformers = getTransformerList(row);
			for(Transformer transformer : transformers){

				for(Feature feature : features){
					encoder.updateType(feature.getName(), transformer.getOpType(), transformer.getDataType());
				}

				features = transformer.encodeFeatures(ids, features, encoder);
			}

			encoder.addRow(ids, features);
		}
	}