调用Facebook SDK登录管理器回调onCancel，并且AccessToken.getCurrentAccessToken（）也返回null

我用我的Android应用程序使用Facebook登录 . 大多数情况下它工作正常，但有时代替OnSuccess方法调用LoginManager回调的OnCancel方法，在这种情况下，AccessToken.getCurrentAccessToken（）也返回null . 以下是我的登录活动的代码片段：

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mGson = new Gson();
        FacebookSdk.sdkInitialize(getApplicationContext());
        callbackManager = CallbackManager.Factory.create();

            LoginManager.getInstance().registerCallback(callbackManager,
                new FacebookCallback<LoginResult>() {
                    @Override
                    public void onSuccess(LoginResult loginResult) {
                        Log.d("Success", "Login--" + loginResult.getAccessToken());
                        accessToken = loginResult.getAccessToken();
                        getUserDetails();
                    }

                    @Override
                    public void onCancel() {
                        if(!NetUtils.isNetworkConnected(getApplicationContext())) {
                            Toast.makeText(FBLoginActivity.this, getString(R.string.error_msg_no_network),
                                    Toast.LENGTH_SHORT).show();
                        } else {
                            accessToken = AccessToken.getCurrentAccessToken();
                            if(accessToken == null) {
                                Toast.makeText(FBLoginActivity.this, "Login unSuccessful..Please contact developer... ", Toast.LENGTH_LONG).show();
                            } else {
                                getUserDetails();
                            }
                        }
                    }

                    @Override
                    public void onError(FacebookException exception) {
                        accessToken = AccessToken.getCurrentAccessToken();
                        if(accessToken == null) {
                            Toast.makeText(FBLoginActivity.this, exception.getMessage(), Toast.LENGTH_LONG).show();
                        } else {
                            getUserDetails();
                        }
                    }
                });

        // Ensure that our profile is up to date
//        Profile.fetchProfileForCurrentAccessToken();
        setContentView(R.layout.activity_login_fb);
        mLayout = (RelativeLayout) findViewById(R.id.MainLayoutLogin);
        fbLoginButton = (ImageView) findViewById(R.id.FBLoginButton);
        fbLoginButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                LoginManager.getInstance().logInWithReadPermissions(FBLoginActivity.this, Arrays.asList("public_profile, email, user_birthday"));
            }
        });

        RestAdapter restAdapter = new RestAdapter.Builder()
                .setEndpoint(ApiEndPoints.BASE_URL)
                .setLogLevel(RestAdapter.LogLevel.FULL)
                .build();
        mUserAPI = restAdapter.create(UserAPI.class);
    }