Исключение нулевого указателя в макете ящика

Я получаю null pointer exception в drawerLayout=(DrawerLayout) findViewById(R.layout.drawer_layout);

Пожалуйста, помогите мне. Почему он возвращает исключение нулевого указателя? Я не знаю, что я делаю неправильно.

package com.example.prototype;

import android.app.ActionBar;
import android.app.Activity;
import android.os.Bundle;
import android.support.v4.app.ActionBarDrawerToggle;
import android.support.v4.widget.DrawerLayout;
import android.support.v4.widget.DrawerLayout.DrawerListener;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

public class MainActivity extends Activity  {

    DrawerLayout drawerLayout;      
    ActionBar action_bar;
    ListView navigation_list;
    ActionBarDrawerToggle drawerToggle;
    String items[];

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.drawer_layout);
        initComponents();
        action_bar=getActionBar();
        setNavigationList();
    }

    @SuppressWarnings("deprecation")
    private void setNavigationList() {

        int open=R.string.navigation_open;
        int close=R.string.navigation_close;
        navigation_list.setAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, items));

        drawerToggle=new ActionBarDrawerToggle(this,drawerLayout, R.drawable.ic_menu_white_18dp,open,close){

             public void onDrawerClosed(View view) {
                 super.onDrawerClosed(view);
                 action_bar.show();
             }

             public void onDrawerOpened(View drawerView) {
                 super.onDrawerOpened(drawerView);
                 action_bar.hide();
             }
        };

        if(drawerLayout==null){
            Toast.makeText(getApplicationContext(), "null", Toast.LENGTH_LONG).show();
        } else {
            drawerLayout.setDrawerListener(drawerToggle);
            action_bar.setDisplayHomeAsUpEnabled(true);
            action_bar.setHomeButtonEnabled(true);
        }   

    }

    private void initComponents() {
         // TODO Auto-generated method stub

         drawerLayout=(DrawerLayout) findViewById(R.layout.drawer_layout);
         action_bar=getActionBar();
         navigation_list=(ListView) findViewById(R.id.left_drawer);
         items=getResources().getStringArray(R.array.navigation_drawer);

         if(drawerLayout==null){
            Toast.makeText(getApplicationContext(), "null0", Toast.LENGTH_LONG).show();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }

        if (id == R.id.search_menu) {
            Toast.makeText(getApplicationContext(), "Searched Clicked ", Toast.LENGTH_LONG).show();
            return true;
        }

        if (id == R.id.share_menu) {
            Toast.makeText(getApplicationContext(), "Share Clicked ", Toast.LENGTH_LONG).show();
            return true;
        }

        if (id == R.id.navigation_menu) {
            Toast.makeText(getApplicationContext(), "Navigation Clicked ", Toast.LENGTH_LONG).show();
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

Вот мой Xml file

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout 
     xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

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

         <ListView android:id="@+id/left_drawer"
        android:layout_width="240dp"
        android:layout_height="match_parent"
        android:layout_gravity="start"
        android:choiceMode="singleChoice"
        android:divider="@android:color/transparent"
        android:dividerHeight="5dp"
        android:background="#E1E1E1"/>
</android.support.v4.widget.DrawerLayout>