android studio NullPointerException at addDrawerListener method

I’m trying to build a navigation drawer with few items in it. but it’s getting me error for the addDrawerListener method at code below:

public class MainActivity extends AppCompatActivity {
    private DrawerLayout drawer;

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

        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, toolbar,
                R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener(toggle);
        toggle.syncState();
    }

    @Override
    public void onBackPressed() {
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else{
            super.onBackPressed();
        }
    }
}

here is the AndroidManifest.xml :

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

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.MyApplication">
        <activity android:name=".MainActivity"
            android:theme="@style/Theme.MyApplication.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

this is the menu ( drawer_menu.xml ) which seems to be working just ok

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    tools:showIn = "navigation_view">

    <group android:checkableBehavior="single">

        <item android:id="@+id/nav_people"
            android:icon="@drawable/ic_people"
            android:title="@string/people" />
        <item android:id="@+id/nav_goods"
            android:icon="@drawable/ic_goods"
            android:title="@string/goods" />
        <item android:id="@+id/nav_delivery"
            android:icon="@drawable/ic_delivery"
            android:title="@string/delivery" />
        <item android:id="@+id/nav_saloon"
            android:icon="@drawable/ic_saloon"
            android:title="@string/saloon" />
        <item android:id="@+id/nav_setting"
            android:icon="@drawable/ic_setting"
            android:title="@string/setting" />
        <item android:id="@+id/nav_help"
            android:icon="@drawable/ic_help"
            android:title="@string/help" />
        <item android:id="@+id/nav_update"
            android:icon="@drawable/ic_update"
            android:title="@string/update" />
        <item android:id="@+id/nav_about_us"
            android:icon="@drawable/ic_about_us"
            android:title="@string/about_us" />
        <item android:id="@+id/nav_quit"
            android:icon="@drawable/ic_exit"
            android:title="@string/exit" />

    </group>
</menu>

there’s also a header ( nav_header.xml ) which has a small picture and two textviews

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@color/nav_header"
    android:gravity="bottom"
    android:orientation="vertical"
    android:padding="16dp"
    android:theme="@style/ThemeOverlay.AppCompat.Dark">

    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@mipmap/ic_launcher_round" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:paddingTop="8dp"
        android:text="@string/strong_text"
        android:textAppearance="@style/TextAppearance.AppCompat.Body1" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/weak_text" />

</LinearLayout>

here is the themes.xml file. the tutorial which I’m trying to follow is a little outdated and has styles.xml file instead of themes.xml but I’m not sure. Im copy-pasting the <style name="Theme.MyApplication.NoActionBar"> part in themes.xml (night)

<resources xmlns:tools="http://schemas.android.com/tools">
    <!-- Base application theme. -->
    <style name="Theme.MyApplication" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
        <!-- Primary brand color. -->
        <item name="colorPrimary">@color/purple_500</item>
        <item name="colorPrimaryVariant">@color/purple_700</item>
        <item name="colorOnPrimary">@color/white</item>
        <!-- Secondary brand color. -->
        <item name="colorSecondary">@color/teal_200</item>
        <item name="colorSecondaryVariant">@color/teal_700</item>
        <item name="colorOnSecondary">@color/black</item>
        <!-- Status bar color. -->
        <item name="android:statusBarColor" tools:targetApi="l">?attr/colorPrimaryVariant</item>
        <!-- Customize your theme here. -->
    </style>

    <style name="Theme.MyApplication.NoActionBar">
        <item name="windowActionBar">false</item>
        <item name="windowNoTitle">true</item>
        <item name="android:statusBarColor">@android:color/transparent</item>
    </style>

</resources>