Wie kann ich den Text eines TextView in der Kopfzeile der Navigationsleiste ändern?

Ich möchte den Text einer TextView in der Kopfzeile der Navigationsleiste ändern. Aber ich bekomme diesen Fehler:

Java.lang.NullPointerException: Versuch, die virtuelle Methode 'void .__' aufzurufen. Android.widget.TextView.setText (Java.lang.CharSequence) 'auf einem Nullwert Objektreferenz

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<Android.support.v4.widget.DrawerLayout xmlns:Android="http://schemas.Android.com/apk/res/Android"
    xmlns:app="http://schemas.Android.com/apk/res-auto"
    xmlns:tools="http://schemas.Android.com/tools"
    Android:id="@+id/drawer_layout"
    Android:layout_width="match_parent"
    Android:layout_height="match_parent"
    Android:fitsSystemWindows="true"
    tools:openDrawer="start">

    <LinearLayout
        Android:layout_width="match_parent"
        Android:layout_height="match_parent"
        Android:orientation="vertical">

        <include
            layout="@layout/app_bar_main"
            Android:layout_width="match_parent"
            Android:layout_height="wrap_content" />

        <FrameLayout
            Android:id="@+id/frame_container"
            Android:layout_width="match_parent"
            Android:layout_height="match_parent" />

    </LinearLayout>

    <Android.support.design.widget.NavigationView
        Android:id="@+id/nav_view"
        Android:layout_width="wrap_content"
        Android:layout_height="match_parent"
        Android:layout_gravity="start"
        Android:fitsSystemWindows="true"
        app:headerLayout="@layout/nav_header_main"
        app:menu="@menu/activity_main_drawer" />

</Android.support.v4.widget.DrawerLayout>

nav_header_main

<?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="@dimen/nav_header_height"
    Android:background="@drawable/slide_nav_bar"
    Android:gravity="bottom"
    Android:orientation="vertical"
    Android:paddingBottom="@dimen/activity_vertical_margin"
    Android:paddingLeft="@dimen/activity_horizontal_margin"
    Android:paddingRight="@dimen/activity_horizontal_margin"
    Android:paddingTop="@dimen/activity_vertical_margin"
    Android:theme="@style/ThemeOverlay.AppCompat.Dark">

    <ImageView
        Android:id="@+id/tvHeaderIcon"
        Android:layout_width="60dp"
        Android:layout_height="60dp"
        Android:paddingTop="@dimen/nav_header_vertical_spacing"
        Android:src="@Android:drawable/sym_def_app_icon" />

    <TextView
        Android:layout_width="match_parent"
        Android:layout_height="wrap_content"
        Android:paddingTop="@dimen/nav_header_vertical_spacing"
        Android:text="@string/app_name"
        Android:textAppearance="@style/TextAppearance.AppCompat.Body1" />

    <TextView
        Android:id="@+id/tvHeaderName"
        Android:layout_width="wrap_content"
        Android:layout_height="wrap_content" />

</LinearLayout>

MainActivity.Java

TextView tvHeaderName;

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

    tvHeaderName= (TextView) findViewById(R.id.tvHeaderName);
    tvHeaderName.setText("Saly");    
}

Wie kann ich das machen?