output: #fixed code
                                @ApiMethod(name = "processSignResponse")
    public List<String> processSignResponse(
        @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 authenticatorData = object.get("authenticatorData").getAsString();
      String signature = object.get("signature").getAsString();
  
      AuthenticatorAssertionResponse assertion =
          new AuthenticatorAssertionResponse(clientDataJSON, authenticatorData, signature);
  
      // TODO
      String credentialId = BaseEncoding.base64Url().encode(
          assertion.getAuthenticatorData().getAttData().getCredentialId());
      String type = null;
      String session = null;
  
      PublicKeyCredential cred = new PublicKeyCredential(credentialId, type,
          BaseEncoding.base64Url().decode(credentialId), assertion);
  
      try {
        U2fServer.verifyAssertion(cred, user.getEmail(), session);
      } 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;
    }
input: #vulnerable code 
                                @ApiMethod(name = "processSignResponse")
    public List<String> processSignResponse(
        @Named("responseData") String responseData, User user)
        throws OAuthRequestException, ResponseException {
      if (user == null) {
        throw new OAuthRequestException("User is not authenticated");
      }
  
      AuthenticatorAssertionResponse assertion =
          new AuthenticatorAssertionResponse(responseData);
  
      // TODO
      String credentialId = BaseEncoding.base64Url().encode(
          assertion.getAuthenticatorData().getAttData().getCredentialId());
      String type = null;
      String session = null;
  
      PublicKeyCredential cred = new PublicKeyCredential(credentialId, type,
          BaseEncoding.base64Url().decode(credentialId), assertion);
  
      try {
        U2fServer.verifyAssertion(cred, user.getEmail(), session);
      } 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 14 
                              #vulnerability type NULL_DEREFERENCE