SupportMapFragment with navigationDrawer - android-fragments

Question

I have an activity that extends ActionBarActivity and has a navigation drawer. This is the xml layout of the activity:
<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"
tools:context=".MainActivity">
<FrameLayout
android:id="#+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
<fragment android:id="#+id/navigation_drawer"
android:layout_width="#dimen/navigation_drawer_width"
android:layout_height="match_parent"
android:layout_gravity="start"
android:name="mypackage.utils.NavigationDrawerFragment"
tools:layout="#layout/fragment_navigation_drawer"/>
</android.support.v4.widget.DrawerLayout>
And then I have a fragment with the map:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/fragment_container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity$PlaceholderFragment">
<fragment
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="#+id/map"
tools:context=".MainActivity"
android:name="com.google.android.gms.maps.SupportMapFragment"/>
<ImageButton
android:onClick="showUserPosition"
android:background="#drawable/selector"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
</RelativeLayout>
When I choose the Map section from the drawer the map fragment is attached to the container and I need to instanciate a GoogleMap object. I tried with:
mMap = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map)).getMap();
and with
mMap = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.container)).getMap();
but always got NullPointerException.. I know it's not a problem of execution time because I even tried to instantiate the GoogleMap in a botton onClick() method that I pressed long after the map showed. How can I get my map reference?

Answer 1

Finally I managed to get it work with these 3 lines of code:
Fragment f = getSupportFragmentManager().findFragmentById(R.id.container);
SupportMapFragment mapFragment = (SupportMapFragment) f.getChildFragmentManager().findFragmentById(R.id.map);
GoogleMap mMap = mapFragment.getMap();