Andorid上下文菜单

public boolean onContextItemSelected(MenuItem item) {
        AdapterView.AdapterContextMenuInfo info = (AdapterView.AdapterContextMenuInfo) item.getMenuInfo();
        String string=null;
        switch (item.getItemId()){
            case R.id.openfile://open的子菜单不能获取postion
                Log.i(TAG,"-->"+info.toString() );
                string = arrayAdapter.getItem(info.position);
                Toast.makeText(MainActivity.this,string,Toast.LENGTH_LONG).show();
                break;
            case R.id.open://父菜单正常响应可以获取postion
                Log.i(TAG,"-->"+info.toString() );
                Log.i(TAG,"-->"+info.position );
                string = arrayAdapter.getItem(info.position);
                Toast.makeText(MainActivity.this,string,Toast.LENGTH_LONG).show();
                break;
          
        }
        return super.onContextItemSelected(item);
    }在一个ListView里响应了 菜单
进入子菜单后,子菜单在 onContextItemSelected 响应后 无法获取 listview中的postion  错误信息如下
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
        at acgn.tech.menu.MainActivity.onContextItemSelected(MainActivity.java:46)
        at android.app.Activity.onMenuItemSelected(Activity.java:3616)
        at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:436)
        at android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:196)
        at android.support.v7.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:109)
        at com.android.internal.policy.PhoneWindow$PhoneWindowMenuCallback.onMenuItemSelected(PhoneWindow.java:3744)
        at com.android.internal.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:776)
        at com.android.internal.view.menu.SubMenuBuilder.dispatchMenuItemSelected(SubMenuBuilder.java:82)
        at com.android.internal.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:148)
        at com.android.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:923)
        at com.android.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:913)
        at com.android.internal.view.menu.MenuPopup.onItemClick(MenuPopup.java:128)
        at android.widget.AdapterView.performItemClick(AdapterView.java:318)
        at android.widget.AbsListView.performItemClick(AbsListView.java:1192)
        at android.widget.AbsListView$PerformClick.run(AbsListView.java:3184)
        at android.widget.AbsListView$3.run(AbsListView.java:4166)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:201)
        at android.app.ActivityThread.main(ActivityThread.java:6806)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)
布局文件没有问题,就是在上下文菜单中,子菜单不能获取 listview的 postion,请求大神们指点迷津