Viewpage null pointer przy ustawianiu adaptera-prośba o sprawdzenie kodu

O to moja klasa z Activity

public class CalendarActivity extends
        FragmentActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_calendar);
         FragmentManager fm =getSupportFragmentManager();
        CalendarFragment calendarFragment =new CalendarFragment();
        CalendarViewpagerAdapter calendarViewpagerAdapter = new CalendarViewpagerAdapter(fm, calendarFragment);
        ViewPager pager = (ViewPager) findViewById(R.id.calendarViewPager);
        pager.setAdapter(calendarViewpagerAdapter);
    }
}

A tu mój layout do niej

<LinearLayout 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:orientation="vertical">

    <android.support.v4.view.ViewPager
        android:id="@+id/calendarViewpager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

I błąd, który się pojawia
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference
I nie mam pojęcia skąd się bierze.Wszelka pomoc będzie mile widziana!