OptionMenu android特有的凌空错误

Question

我正在为一所大学的学术管理系统制作一个简单的应用程序 最初，我有一个MainActivity，其中包含从xml文件加载的选项菜单，以及一个被调用到API的请求，两者都是成功的。 然后，我向MainActivity添加了一个onCreateOptionsMenu（）方法。这导致应用程序崩溃，并且调试消息表明对同一API的请求发出错误。

我的onCreateOptionsMenu（）代码是

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        Log.d(TAG,"Entered onCreateOptionsMenu");

        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.drawermenu, menu);
        //super.onCreateOptionsMenu(menu);

        MenuItem courseitem = menu.findItem(R.id.courses);
        SubMenu coursemenu = courseitem.getSubMenu();

        //The CourseObject contained Json of user and courses..So courses now has only course specific json..
        JSONArray courses = null;
        if( SessionManager.getCourseData()==null){
            Log.d(TAG,"coursedata is empty");
            callCourse();
        }

        try {
            courses = (JSONArray)SessionManager.getCourseData().get("courses");
        } 
        catch (JSONException e) {
            e.printStackTrace();
        }
        String coursecode = "";
        int courseId = 0;
        //For Loop for adding courses to submenu
        for (int i = 0; i < courses.length(); i++){
            JSONObject course = null;
            try
            {
                course = courses.getJSONObject(i);
                //Log.d(TAG, course.toString());
                coursecode = (String) course.get("code");
                courseId = (int) course.get("id");
            } 
            catch (JSONException e)
            {
                e.printStackTrace();
            }
            coursemenu.add(Menu.NONE,courseId,Menu.NONE,coursecode);
        }


        return true;
    }

我的截击请求的代码是

//this is entered when the user has just logged in...
Log.d(TAG, "calling course api");
RequestQueue requestQueue = Volley.newRequestQueue(this,       SessionManager.httpStack);
//requestqueue is made using http-stack as we need to check the sessions of the logged in user
JsonObjectRequest courseRequest = new JsonObjectRequest
    (Request.Method.GET, course_url, null, new Response.Listener<JSONObject> () {

        @Override
        public void onResponse(JSONObject response) {
            Log.d(TAG, response.toString());
            //pDialog.setMessage("Response: "+ response.toString());
            //0th element of the course object now has the response
            courseobject[0] = response;
            Log.d(TAG,"success");
            //So the dialog box is hid now...
            //pDialog.hide();
            if (courseobject[0] != null)
            {
                //Also the Course data is put in a hash map of the Session Manager Preferences so that we dont need to call this API again...
                SessionManager.setCourseData(courseobject[0]);
                Log.d(TAG, "setting course data in cookie");

                // Setup UI elements
                setup();

                // Call home fragment - course buttons created there
                FragmentTransaction xfragmentTransaction = mFragmentManager.beginTransaction();
                xfragmentTransaction.replace(R.id.containerView,new FragmentHome()).commit();

                callGrades();
            }
            else
            {
                //The Response is null and the user is not registered in any course till now...
                Toast.makeText(getApplicationContext(), "Not registered for any course", LENGTH_LONG).show();
            }
        }
    }, new Response.ErrorListener() {

        @Override
        public void onErrorResponse(VolleyError error)
        {
            VolleyLog.d(TAG, "Error: " + error.getMessage());
            //pDialog.setMessage(error.getMessage());
            Toast.makeText(getApplicationContext(), "Failed to fetch course data", LENGTH_LONG).show();
            //pDialog.setMessage(error.getCause().toString());
            //pDialog.hide();
        }
    });

//课程申请现已添加到请求队列中... requestQueue.add（courseRequest）;

日志中的错误消息是（格式化后）

Entered onCreateOptionsMenu
coursedata is empty
http://10.192.44.89:8000/courses/list.json
calling course api
AndroidRuntime: Shutting down VM
I/System.out: Thread-31118(ApacheHTTPLog):isSBSettingEnabled false
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.quantumcoder.moodleplus, PID: 30013
java.lang.NullPointerException: Attempt to invoke virtual method java.lang.Object org.json.JSONObject.get(java.lang.String) on a null object reference
at com.example.quantumcoder.moodleplus.MainActivity.onCreateOptionsMenu(MainActivity.java:290)
at android.app.Activity.onCreatePanelMenu(Activity.java:2959)
at android.support.v4.app.FragmentActivity.onCreatePanelMenu(FragmentActivity.java:298)
at android.support.v7.internal.view.WindowCallbackWrapper.onCreatePanelMenu(WindowCallbackWrapper.java:85)
at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.onCreatePanelMenu(AppCompatDelegateImplBase.java:243)
at android.support.v7.internal.view.WindowCallbackWrapper.onCreatePanelMenu(WindowCallbackWrapper.java:85)
at android.support.v7.internal.app.ToolbarActionBar.populateOptionsMenu(ToolbarActionBar.java:448)
at android.support.v7.internal.app.ToolbarActionBar$1.run(ToolbarActionBar.java:65)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6145)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

我动态创建项目的drawermenu.xml代码是

    <item
        android:title="courses"
        android:id="@+id/courses">
        <menu >                              <!-- add courses here -->
            <item
                android:title="course1"
                android:icon="@drawable/courses"
                android:id="@+id/nav_item_courses"/>
            <item
                android:title="course2"
                android:icon="@drawable/bin"/>
        </menu>
    </item>

任何帮助将不胜感激。谢谢！