问Android: GetMap() ==空在DialogFragment中

EN

public class DialogMapa extends DialogFragment{

    private SupportMapFragment fragment;

    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.layout_dialog_mapa, container,false);
        getDialog().requestWindowFeature(Window.FEATURE_NO_TITLE);

        SupportMapFragment fragment = new SupportMapFragment();
        FragmentTransaction transaction = getChildFragmentManager().beginTransaction();
        transaction.add(R.id.map, fragment).commit();    

        GoogleMap mapa = null;
        try {
             mapa =fragment.getMap();
        } catch (Exception e) {
             e.printStackTrace();
        }

        if (mapa==null) Toast.makeText(getActivity(), "NULL", Toast.LENGTH_SHORT).show();


    return view;
    }

}

<?xml version="1.0" encoding="utf-8"?>

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="0dp" >

    <FrameLayout
        android:id="@+id/map"
        android:layout_width="match_parent"
        android:layout_height="match_parent" >
    </FrameLayout>

</RelativeLayout>