#fixed code
                              @ApiMethod(name = "processRegistrationResponse")
  public List<String> processRegistrationResponse(
      @Named("responseData") String responseData, User user)
      throws OAuthRequestException, ResponseException {
    if (user == null) {
      throw new OAuthRequestException("User is not authenticated");
    }

    Gson gson = new Gson();
    JsonElement element = gson.fromJson(responseData, JsonElement.class);
    JsonObject object = element.getAsJsonObject();
    String clientDataJSON = object.get("clientDataJSON").getAsString();
    String attestationObject = object.get("attestationObject").getAsString();
    
    AuthenticatorAttestationResponse attestation =
        new AuthenticatorAttestationResponse(clientDataJSON, attestationObject);

    // TODO
    String credentialId = BaseEncoding.base64Url().encode(
        attestation.getAttestationObject().getAuthenticatorData().getAttData().getCredentialId());
    String type = null;
    String session = null;
    PublicKeyCredential cred = new PublicKeyCredential(credentialId, type,
        BaseEncoding.base64Url().decode(credentialId), attestation);

    try {
      switch (cred.getAttestationType()) {
        case FIDOU2F:
          U2fServer.registerCredential(cred, user.getEmail(), session, Constants.APP_ID);
          break;
        case ANDROIDSAFETYNET:
          AndroidSafetyNetServer.registerCredential(
              cred, user.getEmail(), session, Constants.APP_ID);
          break;
        default:
          // This should never happen.
      }
    } catch (ServletException e) {
      // TODO
    }

    Credential credential = new Credential(cred);
    credential.save(user.getEmail());

    List<String> resultList = new ArrayList<String>();
    resultList.add(credential.toJson());
    return resultList;
  }
#vulnerable code 
                              @ApiMethod(name = "processRegistrationResponse")
  public List<String> processRegistrationResponse(
      @Named("responseData") String responseData, User user)
      throws OAuthRequestException, ResponseException {
    if (user == null) {
      throw new OAuthRequestException("User is not authenticated");
    }

    Gson gson = new Gson();
    JsonElement element = gson.fromJson(responseData, JsonElement.class);
    AuthenticatorAttestationResponse attestation =
        new AuthenticatorAttestationResponse(element);

    // TODO
    String credentialId = BaseEncoding.base64Url().encode(
        attestation.getAttestationObject().getAuthenticatorData().getAttData().getCredentialId());
    String type = null;
    String session = null;
    PublicKeyCredential cred = new PublicKeyCredential(credentialId, type,
        BaseEncoding.base64Url().decode(credentialId), attestation);

    try {
      switch (cred.getAttestationType()) {
        case FIDOU2F:
          U2fServer.registerCredential(cred, user.getEmail(), session, Constants.APP_ID);
          break;
        case ANDROIDSAFETYNET:
          AndroidSafetyNetServer.registerCredential(
              cred, user.getEmail(), session, Constants.APP_ID);
          break;
        default:
          // This should never happen.
      }
    } catch (ServletException e) {
      // TODO
    }

    Credential credential = new Credential(cred);
    credential.save(user.getEmail());

    List<String> resultList = new ArrayList<String>();
    resultList.add(credential.toJson());
    return resultList;
  } 
                            #location 16 
                            #vulnerability type NULL_DEREFERENCE