output: #fixed code
                                  @Override
      public String validateRecord(Record record) {
  
          String error = super.validateRecord(record);
          if (error == null){//no errors after applying declared validators on each field => all fields are valid
  
              //add custom validation : field 2 content must start with field's 1 content
              final String content1 = record.getFieldContentByIndex(1);
              final String content2 = record.getFieldContentByIndex(2);
              if (!content2.startsWith(content1))
                  return "field 2 content [" + content2 + "] must start with field's 1 content [" + content1 + "]";
          }
          return null;
      }
input: #vulnerable code 
                                  @Override
      public String validateRecord(Record record) {
  
          String error = super.validateRecord(record);
          if (error.length() == 0){//no errors after applying declared validators on each field => all fields are valid
  
              //add custom validation : field 2 content must starts with field 1 content
              final String content1 = record.getFieldContentByIndex(1);
              final String content2 = record.getFieldContentByIndex(2);
              if (!content2.startsWith(content1))
                  return "field 2 content [" + content2 + "] must start with field 1 content [" + content1 + "]";
          }
          return "";
      } 
                              #location 5 
                              #vulnerability type NULL_DEREFERENCE