如何将文本Volley键setText到NavigationHeader的TextView中？

Question

我需要将Volley键放到NavigationHeader内的TextView中，但是它一直给我这个错误

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
        at com.example.a6thacanect.MainActivity.onCreate(MainActivity.java:106)

这是我用于NavigationHeader的全部代码。

NavigationView navigationView = (NavigationView) findViewById (R.id.nav_view);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle (
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.addDrawerListener (toggle);
        toggle.syncState ();
        navigationView.setNavigationItemSelectedListener (this);
        View header = navigationView.getHeaderView (0);
        final SharedPreferences shared = (getSharedPreferences ("Mypref", Context.MODE_PRIVATE));
        String username = shared.getString("username", "");
        String nickname = shared.getString("nickname", "");
        final TextView username1 = findViewById (R.id.username);
        final TextView nickname1 = findViewById (R.id.nickname);
        username1.setText(username);
        nickname1.setText(nickname);
        TextView headUsername = (TextView) findViewById (R.id.usernameHeader);
        TextView headNickname = (TextView) findViewById (R.id.nicknameHeader);
        headUsername.setText ((CharSequence) username1);
        headNickname.setText ((CharSequence) nickname1);

(MainActivity.java:106)是username1.setText(username);

现在，NavigationHeader的初始显示如下：

Android Studio android.studio@android.com

但我希望它是

"(R.id.nickname);" "(R.id.username);"

示例：

JohnSmith65 @johnsmith65

希望您能帮助我。谢谢。