使用facebook android studio登录java.lang.NullPointerException

在使用facebook登录时，我一次又一次地遇到这个错误。我无法理解。请帮忙

这是我的登录活动代码：

public class LoginActivity extends AppCompatActivity {

    private CallbackManager callbackManager;
    private AccessTokenTracker accessTokenTracker;
    private ProfileTracker profileTracker;

    //Facebook login button
    private FacebookCallback<LoginResult> callback = new FacebookCallback<LoginResult>() {
        @Override
        public void onSuccess(LoginResult loginResult) {
            Profile profile = Profile.getCurrentProfile();
            nextActivity(profile);
        }
        @Override
        public void onCancel() {        }
        @Override
        public void onError(FacebookException e) {      }
    };


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        FacebookSdk.sdkInitialize(getApplicationContext());
        callbackManager = CallbackManager.Factory.create();
        accessTokenTracker = new AccessTokenTracker() {
            @Override
            protected void onCurrentAccessTokenChanged(AccessToken oldAccessToken, AccessToken currentAccessToken) {

            }
        };

        profileTracker = new ProfileTracker() {
            @Override
            protected void onCurrentProfileChanged(Profile oldProfile, Profile newProfile) {
                nextActivity(newProfile);
            }
        };
        accessTokenTracker.startTracking();
        profileTracker.startTracking();
        LoginButton loginButton = (LoginButton)findViewById(R.id.login_button);
        callback = new FacebookCallback<LoginResult>() {
            @Override
            public void onSuccess(LoginResult loginResult) {
                AccessToken accessToken = loginResult.getAccessToken();
                Profile profile = Profile.getCurrentProfile();
                nextActivity(profile);
                Toast.makeText(getApplicationContext(), "Logging in...", Toast.LENGTH_SHORT).show();    }

            @Override
            public void onCancel() {
            }

            @Override
            public void onError(FacebookException e) {
            }
        };
        loginButton.setReadPermissions("user_friends");
        loginButton.registerCallback(callbackManager, callback);
    }

    @Override
    protected void onResume() {
        super.onResume();
        //Facebook login
        Profile profile = Profile.getCurrentProfile();
        nextActivity(profile);
    }

    @Override
    protected void onPause() {

        super.onPause();
    }

    protected void onStop() {
        super.onStop();
        //Facebook login
        accessTokenTracker.stopTracking();
        profileTracker.stopTracking();
    }

    @Override
    protected void onActivityResult(int requestCode, int responseCode, Intent intent) {
        super.onActivityResult(requestCode, responseCode, intent);
        //Facebook login
        callbackManager.onActivityResult(requestCode, responseCode, intent);

    }

    private void nextActivity(Profile profile){
        if(profile != null){
            Intent main = new Intent(LoginActivity.this, MainActivity.class);
            main.putExtra("name", profile.getFirstName());
            main.putExtra("surname", profile.getLastName());
            main.putExtra("imageUrl", profile.getProfilePictureUri(200,200).toString());
            startActivity(main);
        }
    }
}

---

您尚未将活动声明到清单文件中

 <activity
        android:name="LoginActivity"/>

无论何时创建新的活动类文件，都必须将其声明为清单文件

 <activity
        android:name="LoginActivity"/>

---

您尚未将活动声明到清单文件中

 <activity
        android:name="LoginActivity"/>

无论何时创建新的活动类文件，都必须将其声明为清单文件

 <activity
        android:name="LoginActivity"/>

这是我的清单代码

这是我的清单代码