Вот мой код в LoginActivity:

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

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }

    loginButton = (Button)findViewById(R.id.loginButton);

    loginButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            onLoginButtonClicked();
        }
    });

    // Check if there is a currently logged in user
    // and they are linked to a Facebook account.
    ParseUser currentUser = ParseUser.getCurrentUser();
    if ((currentUser != null) && ParseFacebookUtils.isLinked(currentUser)) {
        // Go to the main activity
        showHomeActivity();
    }
}

во фрагменте_login.xml

<Button
    android:id="@+id/loginButton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerHorizontal="true"
    android:layout_centerVertical="true"
    android:text="@string/login_button" />

И это журнал, когда я запускаю приложение:

E/AndroidRuntime(6647): FATAL EXCEPTION: main
E/AndroidRuntime(6647): Process: com.moostachestudio.drinkitapp, PID: 6647
E/AndroidRuntime(6647): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.moostachestudio.drinkitapp/com.moostachestudio.drinkitapp.LoginActivity}: java.lang.NullPointerException
E/AndroidRuntime(6647):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
E/AndroidRuntime(6647):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
E/AndroidRuntime(6647):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
E/AndroidRuntime(6647):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
E/AndroidRuntime(6647):     at android.os.Handler.dispatchMessage(Handler.java:102)
E/AndroidRuntime(6647):     at android.os.Looper.loop(Looper.java:136)
E/AndroidRuntime(6647):     at android.app.ActivityThread.main(ActivityThread.java:5017)
E/AndroidRuntime(6647):     at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(6647):     at java.lang.reflect.Method.invoke(Method.java:515)
E/AndroidRuntime(6647):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
E/AndroidRuntime(6647):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
E/AndroidRuntime(6647):     at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime(6647): Caused by: java.lang.NullPointerException
E/AndroidRuntime(6647):     at com.moostachestudio.drinkitapp.LoginActivity.onCreate(LoginActivity.java:45)
E/AndroidRuntime(6647):     at android.app.Activity.performCreate(Activity.java:5231)
E/AndroidRuntime(6647):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
E/AndroidRuntime(6647):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
E/AndroidRuntime(6647):     ... 11 more

Проблема в том, что loginButton равен NULL, но я не знаю почему, потому что я вызываю findViewById после setContentView!