output: #fixed code
                                  public FieldInfo propertyField(String propertyName) {
          if (propertyFields == null) {
              initPropertyFields();
          }
          return propertyFields.get(propertyName.toLowerCase());
      }
input: #vulnerable code 
                                  public FieldInfo propertyField(String propertyName) {
          if (propertyFields == null) {
  
              if (propertyFields == null) {
                  Collection<FieldInfo> fieldInfos = propertyFields();
                  propertyFields = new HashMap<>(fieldInfos.size());
                  for (FieldInfo fieldInfo : fieldInfos) {
  
                      propertyFields.put(fieldInfo.property().toLowerCase(), fieldInfo);
                  }
              }
          }
          return propertyFields.get(propertyName.toLowerCase());
      } 
                              #location 9 
                              #vulnerability type NULL_DEREFERENCE