title: Get subview of loaded view (group inside a drawer)
body: <p>I have a drawer with actions in a group, file a list of files I want to dynamically put in another group. My main activity has this layout:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;android.support.v4.widget.DrawerLayout
      xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto"
      xmlns:tools="http://schemas.android.com/tools"
      android:id="@+id/drawer_layout"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:fitsSystemWindows="true"
      tools:openDrawer="start"&gt;
  
      &lt;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"/&gt;
  &lt;/android.support.v4.widget.DrawerLayout&gt;
  </code></pre>
  
  <p>Then the loaded NavigationView is:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;menu xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/test_id"&gt;
      &lt;group android:checkableBehavior="single" android:id="@+id/group_file_list"&gt;
          &lt;item
              android:id="@+id/file1"
              android:icon="@drawable/ic_menu_gallery"
              android:title="File 1"/&gt;
          ...
      &lt;/group&gt;
  
      &lt;item android:title="Actions"&gt;
          &lt;menu&gt;
              &lt;item
                  android:id="@+id/nav_share"
                  android:icon="@drawable/ic_menu_share"
                  android:title="Share"/&gt;
              &lt;item
                  android:id="@+id/nav_send"
                  android:icon="@drawable/ic_menu_send"
                  android:title="Send"/&gt;
          &lt;/menu&gt;
      &lt;/item&gt;
  &lt;/menu&gt;
  </code></pre>
  
  <p>When I try to get the group where the files are to later add the Items dynamically, all of these return null:</p>
  
  <pre><code>findViewById(R.id.group_file_list); // returns null
  
  View navView = findViewById(R.id.nav_view); // works
  navView.findViewById(R.id.group_file_list); // returns null
  navView.findViewById(R.id.test_id); // returns null
  </code></pre>
  
  <p>And yes, I'm calling this after <code>setContentView()</code>. I also tried inside <code>onStart()</code></p>
  
  <p>Any ideas?</p>