#vulnerable code 
                              @Override
  public boolean doTransaction(DB db, Object threadstate) {
    switch (operationchooser.nextString()) {
    case "READ":
      doTransactionRead(db);
      break;
    case "UPDATE":
      doTransactionUpdate(db);
      break;
    case "INSERT":
      doTransactionInsert(db);
      break;
    case "SCAN":
      doTransactionScan(db);
      break;
    default:
      doTransactionReadModifyWrite(db);
    } 

    return true;
  } 
                            #location 3 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              @Override
  public boolean doTransaction(DB db, Object threadstate) {
    String operation = operationchooser.nextString();
    if(operation == null) {
      return false;
    }

    switch (operation) {
    case "READ":
      doTransactionRead(db);
      break;
    case "UPDATE":
      doTransactionUpdate(db);
      break;
    case "INSERT":
      doTransactionInsert(db);
      break;
    case "SCAN":
      doTransactionScan(db);
      break;
    default:
      doTransactionReadModifyWrite(db);
    }

    return true;
  }