用另一个替换一个导航视图

Question

我想用另一个菜单替换一个菜单，在这个视图中我有物品回家，产品等..，同时点击产品它打开另一个包含差异产品的菜单。在下面的代码中我试图替换另一个菜单，但我得到空指针异常，但我想知道在点击导航项目我如何切换到另一个导航视图？ 任何人都可以建议在这种情况下做什么？I have attached logcat snapshot.

public void selectDrawerItem(MenuItem menuItem)
{
    // Create a new fragment and specify the fragment to show based on nav item clicked
    Fragment fragment = null;
    Class fragmentClass;
    switch(menuItem.getItemId())
    {
        case R.id.menu_home:
            fragmentClass = A.class;
            break;
        case R.id.menu_products:
            NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
            navigationView.getMenu().clear();
            navigationView.inflateMenu(R.menu.menu_products);
            break;
        case R.id.menu_B:
            fragmentClass = B.class;
            break;
        case R.id.menu_C:
            fragmentClass = C.class;
            break;
        case R.id.menu_D:
            fragmentClass = d.class;
            break;
        case R.id.menu_E:
            fragmentClass = E.class;
            break;
        case R.id.menu_F:
            fragmentClass = F.class;
            break;

        default:
            fragmentClass = A.class;
    }


      try
    {
        fragment = (Fragment) fragmentClass.newInstance();
    } catch (Exception e)
    {
        e.printStackTrace();
    }

    // Insert the fragment by replacing any existing fragment
    FragmentManager fragmentManager = getSupportFragmentManager();
//this is line 155:    
fragmentManager.beginTransaction().replace(R.id.frame, fragment).commit();

    // Highlight the selected item has been done by NavigationView
    menuItem.setChecked(true);
    // Set action bar title
    setTitle(menuItem.getTitle());
    // Close the navigation drawer
    drawer.closeDrawers();

 }
 private void setupDrawerContent(NavigationView navigationView){
    navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener()
            {
                @Override
                public boolean onNavigationItemSelected(MenuItem menuItem)
                {
                    selectDrawerItem(menuItem);
                    return true;
                }
            });
}