#vulnerable code 
                              public Result<AccessToken> accessToken(String clientId, String clientSecret, String grantType,
      String code, String redirectUri) {
    List<NameValuePair> params = new ArrayList<NameValuePair>();
    weibo.addParameter(params, "client_id", clientId);
    weibo.addParameter(params, "client_secret", clientSecret);
    weibo.addParameter(params, "grant_type", grantType);
    if ("authorization_code".equals(grantType)) {
      weibo.addParameter(params, "code", code);
      weibo.addParameter(params, "redirect_uri", redirectUri);
    }
    String result = weibo.post("https://api.weibo.com/oauth2/access_token", params);
    return Result.perse(result, AccessToken.class);
  } 
                            #location 12 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              public Result<AccessToken> accessToken(String clientId, String clientSecret, String grantType,
      String code, String redirectUri) {
    List<NameValuePair> params = new ArrayList<NameValuePair>();
    weibo.addParameter(params, "client_id", clientId);
    weibo.addParameter(params, "client_secret", clientSecret);
    weibo.addParameter(params, "grant_type", grantType);
    if ("authorization_code".equals(grantType)) {
      weibo.addParameter(params, "code", code);
      weibo.addParameter(params, "redirect_uri", redirectUri);
    }
    String result = weibo.post("https://api.weibo.com/oauth2/access_token", params);
    return Result.parse(result, AccessToken.class);
  }