android - getSupportFragmentManager returns null

Question

I'd like to show Google maps in my android app:

public class DisplayMapActivity extends ActionBarActivity {
    private LbsTracker2 lbsTracker2;
    private GoogleMap googleMap;

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

        try {
            this.initilizeMap();

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    protected void onResume() {
        super.onResume();
        initilizeMap();
    }

    private void initilizeMap() {
        if (googleMap == null) {
            MapFragment mapFragment = (MapFragment) getFragmentManager().findFragmentById(R.id.map); // <--- always null! :(
            googleMap = mapFragment.getMap();
        }
    }
    ...
}

But my variable mapFragment is always null inside my method initializeMap() ...

Xml of my activity:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.lbsapp.DisplayMapActivity$PlaceholderFragment" >

    <fragment
            android:id="@+id/map"
            android:name="com.google.android.gms.maps.MapFragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:tag="tag_fragment_map" />

</RelativeLayout>

Manifest of my app:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.lbsapp"
    android:versionCode="1"
    android:versionName="1.0" >

    <permission
        android:name="info.androidhive.googlemapsv2.permission.MAPS_RECEIVE"
        android:protectionLevel="signature" />

    <uses-permission android:name="info.androidhive.googlemapsv2.permission.MAPS_RECEIVE" />

    <uses-sdk
        android:minSdkVersion="14"
        android:targetSdkVersion="19" />

    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="com.google.android.providers.gsf.permission.READ_GSERVICES" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

    <!-- Required to show current location --> 
    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
    <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />

    <!-- Required OpenGL ES 2.0. for Maps V2 -->
    <uses-feature
        android:glEsVersion="0x00020000"
        android:required="true" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.lbsapp.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name="com.example.lbsapp.DisplayLbsActivity"
            android:label="@string/title_activity_display_lbs"
            android:parentActivityName="com.example.lbsapp.MainActivity" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="com.example.lbsapp.MainActivity" />
        </activity>

        <!-- Goolge Maps API Key -->
        <meta-data
            android:name="com.google.android.maps.v2.API_KEY"
            android:value="AIzaSyDnnWGwO8Zfbn5PGKXPF3aspbZTCtJ6j7I" />

        <activity
            android:name="com.example.lbsapp.DisplayMapActivity"
            android:label="@string/title_activity_display_map"
            android:parentActivityName="com.example.lbsapp.MainActivity" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="com.example.lbsapp.MainActivity" />
        </activity>
    </application>    

</manifest>

Answer 1

The problem was there:

setContentView(R.layout.activity_main);

Of course it has to be

setContentView(R.layout.activity_display_map);