Android导航抽屉切换错误

Question

我遇到了几行问题。当我启动应用程序时，它将一直运行，直到我点击导航抽屉中的某些内容然后它将崩溃。对此的任何帮助都会很棒谢谢。

logcat的：

    FATAL EXCEPTION: main
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime: Process: com.infintro.keondrae.myapplication, PID: 3031
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime: java.lang.NullPointerException: Attempt to write to field 'android.support.v4.app.FragmentManagerImpl android.support.v4.app.Fragment.mFragmentManager' on a null object reference
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:414)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:449)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:441)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at com.infintro.keondrae.myapplication.MainActivity.selectDrawerItem(MainActivity.java:121)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at com.infintro.keondrae.myapplication.MainActivity$2.onNavigationItemSelected(MainActivity.java:73)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:150)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.support.v7.internal.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:811)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.support.v7.internal.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:153)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.support.v7.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:958)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:300)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.view.View.performClick(View.java:5198)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.view.View$PerformClick.run(View.java:21147)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.os.Handler.handleCallback(Handler.java:739)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:95)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:148)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5417)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Native Method)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
10-20 23:02:47.497 3031-3031/? E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

MainActivity.java

public class MainActivity extends AppCompatActivity {
private NavigationView navDrawer;
private DrawerLayout drawer;
private Toolbar toolbar;
private ActionBarDrawerToggle drawerToggle;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //Set toolbar
    toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);



    // For the floating button
    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

    //Find drawer layout
    drawer = (DrawerLayout) findViewById(R.id.drawer_layout);

    drawerToggle = setupDrawerToggle();

    drawer.setDrawerListener(drawerToggle);

    //Find nav drawer
    navDrawer = (NavigationView) findViewById(R.id.nav_view);

    //Setup drawer
    setupDrawerContent(navDrawer);
}

private ActionBarDrawerToggle setupDrawerToggle(){
    return new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
}


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

}

public void selectDrawerItem(MenuItem menuItem){
    //Creating new fragment

    Fragment fragment;
    fragment = null;

    Class fragmentClass;
    switch (menuItem.getItemId()){
        case R.id.nav_camara:
            fragmentClass = One.class;
            break;
        case R.id.nav_gallery:
            fragmentClass = Two.class;
            break;
        case R.id.nav_manage:
            fragmentClass = One.class;
            break;
        case R.id.nav_send:
            fragmentClass = Two.class;
            break;
        case R.id.nav_share:
            fragmentClass = One.class;
            break;
        case R.id.nav_slideshow:
            fragmentClass = Two.class;
            break;
        case R.id.nav_view:
            fragmentClass = One.class;
            break;
        default:
            fragmentClass = One.class;
    }
    try{
        fragment = (Fragment) fragmentClass.newInstance();
    }catch (Exception e){
        e.printStackTrace();
    }
    //replacing frag
    FragmentManager fragmentManager = getSupportFragmentManager();
    fragmentManager.beginTransaction()
            .replace(R.id.flContent,fragment)
            .commit();

    //Highlight
    menuItem.setChecked(true);
    setTitle(menuItem.getTitle());
    drawer.closeDrawers();
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml
    //noinspection SimplifiableIfStatement

    switch (item.getItemId()){
        case android.R.id.home:
            drawer.openDrawer(GravityCompat.START);
            return true;
    }

    return super.onOptionsItemSelected(item);
}

protected void onPostCreate(Bundle saveInstanceStare){
    super.onPostCreate(saveInstanceStare);

    drawerToggle.syncState();
}

public void onConfigurationChanged(Configuration newConfig){
    super.onConfigurationChanged(newConfig);

    drawerToggle.onConfigurationChanged(newConfig);
}

}

错误行

.replace(R.id.flContent,fragment)

和

selectDrawerItem(menuItem);

activity_main.xml中

<!-- This LinearLayout represents the contents of the screen  -->
<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <!-- The ActionBar displayed at the top -->
    <include
        layout="@layout/app_bar_main"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <!-- The main content view where fragments are loaded -->
    <FrameLayout
        android:id="@+id/flContent"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

<android.support.design.widget.NavigationView
    android:id="@+id/nav_view"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:layout_gravity="start"
    android:fitsSystemWindows="true"
    app:headerLayout="@layout/nav_header_main"
    app:menu="@menu/activity_main_drawer" />
</android.support.v4.widget.DrawerLayout>

我已经在这里待了几个小时了，我仍然无法弄清楚问题。