android - facebook注销中的NullPointer异常

Question

有人可以帮我解决以下代码中的空指针异常吗

我有以下FoodStarLoginActivity。以下行抛出一个 NullPointer 但我不明白为什么：facebook.logout(FoodStarLoginActivity.this);有人可以帮助我吗？我什至尝试getApplicationcontext()代替我的FoodStarLoginActivity，但没有奏效。

I am getting following exception  :
java.lang.NullPointerException
at android.context.ContextWrapper.getApplicationContext(ContextWrapper.java:100)
at android.webkit.CookieSyncManager.createInstance(CookiesSyncManager.java:96)
at com.facebook.android.Util.clearCookies(Util.java:241)
at com.facebook.android.Facebook.logout(Facebook.java:617)
at ten.mob.socialapp.FoodStarLoginActivity.logout(FoodStarLoginActivity.java:131)

---

package ten.mob.socialapp;

import java.io.FileNotFoundException;
import java.io.IOException;
import java.net.MalformedURLException;

import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.os.Handler;

import ten.mob.socialapp.FacebookConnector;
import ten.mob.socialapp.SessionEvents;
import ten.mob.socialapp.FacebookConnector.LogoutRequestListener;

import com.facebook.android.AsyncFacebookRunner;
import com.facebook.android.AsyncFacebookRunner.RequestListener;
import com.facebook.android.DialogError;
import com.facebook.android.Facebook;
import com.facebook.android.Facebook.DialogListener;
import com.facebook.android.FacebookError;

public class FoodStarLoginActivity extends Activity{

    private static final String faceBookAppId = "*******************";
    private static final String faceBookPermission = "publish_stream";
    static Facebook facebook = new Facebook(faceBookAppId);
    private SharedPreferences mPrefs;
    FacebookConnector facebookConnector ;
    AsyncFacebookRunner asyncRunner;
    private Handler mHandler = new Handler();
    Context context;
    @Override
    public void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        mPrefs = getPreferences(MODE_PRIVATE);
        String access_token = mPrefs.getString("access_token", null);
        long expires = mPrefs.getLong("access_expires", 0);
        if(access_token != null){
            facebook.setAccessToken(access_token);
        }
        if(expires != 0){
            facebook.setAccessExpires(expires);
        }
        if(!facebook.isSessionValid()){
            facebook.authorize(this, new DialogListener(){
                @Override
                public void onComplete(Bundle values){
                    SharedPreferences.Editor editor = mPrefs.edit();
                    editor.putString("access_token", facebook.getAccessToken());
                    editor.putLong("access_expires", facebook.getAccessExpires());
                    editor.commit();
                }
                @Override
                public void onFacebookError(FacebookError error){}
                @Override
                public void onError(DialogError e){}
                @Override
                public void onCancel(){}
            });
        }
        if(facebook.isSessionValid()){
            Intent intent = new Intent(this,SearchMyActivity.class);
            startActivity(intent);
        }
    }

    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data){
        super.onActivityResult(requestCode, resultCode, data);
        facebook.authorizeCallback(requestCode, resultCode, data);
    }

    public void logout(){
        try {
            facebook.logout(FoodStarLoginActivity.this);
        } catch (MalformedURLException e1) {
            e1.printStackTrace();
        } catch (IOException e1) {
            e1.printStackTrace();
        }
        asyncRunner = new AsyncFacebookRunner(facebook);
        System.out.println(asyncRunner);
        asyncRunner.logout(FoodStarLoginActivity.this, new LogoutRequestListener() {
            public void onComplete(String response, Object state) {}

            public void onFileNotFoundException(FileNotFoundException e,Object state) {}

            public void onFacebookError(FacebookError e, Object state) {}
        });
        System.out.println(asyncRunner);
        System.out.println(facebook.isSessionValid());
    }

    private class LogoutRequestListener implements RequestListener {
        public void onComplete(String response, Object state) {
            mHandler.post(new Runnable() {
                public void run() {
                    SessionEvents.onLogoutFinish();
                }
            });
        }
        public void onIOException(IOException e, Object state) {}
        public void onFileNotFoundException(FileNotFoundException e,Object state) {}
        public void onMalformedURLException(MalformedURLException e,Object state) {}
        public void onFacebookError(FacebookError e, Object state) {}
    }
}

public class SearchMyActivity(){

    @Override
    public boolean onOptionsItemSelected(MenuItem item){
        switch (item.getItemId()){
        case R.id.menu_create:
            Intent intent = new Intent(this, CreateRestaurantActivity.class);
            startActivity(intent);
            break;
        case R.id.menu_foodstars:
            intent = new Intent(this, RestaurantList.class);
            startActivity(intent);
            break;
        case R.id.menu_logout:
            FoodStarLoginActivity logoutFoodStar = new FoodStarLoginActivity();
            logoutFoodStar.logout();
            break;
        default: 
            return super.onOptionsItemSelected(item);
        }
        return true;
    }

}