title: SupportManager.getFragmentById returns null
body: <p>hi all I have been stuck with this getFragment by id returning null</p>
  
  <p>xml file</p>
  
  <pre><code>&lt;android.support.v4.widget.DrawerLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  android:id="@+id/drawer_layout"
  android:layout_width="match_parent"
  android:layout_height="match_parent"&gt;
  
  &lt;FrameLayout
      android:id="@+id/container"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      &gt;
  &lt;/FrameLayout&gt;
  
  &lt;fragment
      android:id="@+id/navigation_drawer"
      android:layout_width="wrap_content"
      android:layout_height="match_parent"
      android:layout_gravity="left"
      tools:layout="@layout/fragment_navigation_drawer_list" /&gt;
  &lt;/android.support.v4.widget.DrawerLayout&gt;
  </code></pre>
  
  <p>MainActivity.java</p>
  
  <pre><code>public class MainActivity extends ActionBarActivity {
  private static final String SCREEN_NAME = "MainActivity";
  
  @Optional @InjectView(R.id.tool_bar) Toolbar toolbar;
  //@InjectView(R.id.DrawerLayout) DrawerLayout mDrawerLayout;
  
  @Inject GamerzWikiApplication mGamerzWikiApplication;
  private NavigationDrawerFragment mNavigationDrawerFragment;
  
  android.support.v7.app.ActionBar mActionBar;
  ActionBarDrawerToggle mDrawerToggable;
  
  private ArrayList&lt;NavigationDrawerItem&gt; mNavigationDrawerItemList;
  private DrawerLayout mDrawerLayout;
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
  
      ButterKnife.inject(this);
      ObjectGraphInjector.inject(this);
  
      setSupportActionBar(toolbar);
      setupActionBar();
      setupDrawerLayout();
  
  }
  
  
  @Override
  public boolean onCreateOptionsMenu(Menu menu) {
      // Inflate the menu; this adds items to the action bar if it is present.
      getMenuInflater().inflate(R.menu.menu_main, menu);
      return true;
  }
  
  private void setupActionBar() {
      mActionBar = getSupportActionBar();
  
      //SetDisplayHomeAsUpEnabled =&gt; Set whether home should be displayed as an "up" affordance(objects action/ to perform).
      //mActionBar.setDisplayHomeAsUpEnabled(true);
  
      //Enable or disable the "home" button in the corner of the action bar.
      mActionBar.setHomeButtonEnabled(true);
  }
  
  private void setupDrawerLayout() {
  
      mNavigationDrawerItemList = new ArrayList&lt;NavigationDrawerItem&gt;();
      mNavigationDrawerItemList.add(new NavigationDrawerItem(getString(R.string.title_home), "{fa-home}") {
          @Override
          public void onClick() {
              mActionBar.setTitle(R.string.title_home);
              /*
              getSupportFragmentManager().beginTransaction()
                      .replace(R.id.container, new TopGameFragment())
                      .commit();
              */
          }
      });
  
  
      mNavigationDrawerItemList.add(new NavigationDrawerItem("ユーザ情報の確認・表示", "{fa-user}") {
          @Override
          public void onClick() {
              //startActivity(AccountSettingActivity.createIntent(getApplicationContext()));
          }
      });
  
      mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
      mNavigationDrawerFragment = (NavigationDrawerFragment)
              getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
      mNavigationDrawerFragment.setup(
              R.id.navigation_drawer,
              mNavigationDrawerItemList,
              mDrawerLayout
      );
  
      mActionBar.setTitle(R.string.title_home);
      /*
      getSupportFragmentManager().beginTransaction()
              .replace(R.id.container, new TopGameFragment())
              .commit();*/
  
  }
  
  @Override
  public void onAttachFragment(Fragment fragment) {
      // TODO Auto-generated method stub
      super.onAttachFragment(fragment);
  
      Toast.makeText(getApplicationContext(), String.valueOf(fragment.getId()), Toast.LENGTH_SHORT).show();
  
  }
  
  @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.
      int id = item.getItemId();
  
      //noinspection SimplifiableIfStatement
      if (id == R.id.action_settings) {
          return true;
      }
  
      return super.onOptionsItemSelected(item);
  }
  </code></pre>
  
  <p>}</p>
  
  <p>This is the exception thrown in the logcat it refers to <code>mNavigationDrawerFragment = (NavigationDrawerFragment)
                  getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);</code></p>
  
  <pre><code> Caused by: java.lang.NullPointerException
          at com.atfreaks.gamerzwiki.MainActivity.setupDrawerLayout(MainActivity.java:101)
          at com.atfreaks.gamerzwiki.MainActivity.onCreate(MainActivity.java:54)
  </code></pre>
  
  <p>I don't know why it returns null . can anyone have idea why this happens? thanks! I included the whole mainActivity.java</p>