output: #fixed code
                                @Override
    public boolean doTransaction(DB db, Object threadstate) {
      String operation = operationchooser.nextString();
      if (operation == null) {
        return false;
      }
  
      switch (operation) {
      case "UPDATE":
        doTransactionUpdate(db);
        break;
      case "INSERT":
        doTransactionInsert(db);
        break;
      case "DELETE":
        doTransactionDelete(db);
        break;
      default:
        doTransactionRead(db);
      }
      return true;
    }
input: #vulnerable code 
                                @Override
    public boolean doTransaction(DB db, Object threadstate) {
      switch (operationchooser.nextString()) {
      case "UPDATE":
        doTransactionUpdate(db);
        break;
      case "INSERT":
        doTransactionInsert(db);
        break;
      case "DELETE":
        doTransactionDelete(db);
        break;
      default:
        doTransactionRead(db);
      }
      return true;
    } 
                              #location 3 
                              #vulnerability type NULL_DEREFERENCE