在偏好片段中使用SeekBar的自定义首选项

Question

我正在尝试设置SeekBar来控制Preferences内的音乐流量。一切似乎都运行正常但是当我访问设置屏幕时，我得到一个例外，说明我试图在seekbar.setMax(int)上拨打NullObjectReference。我不明白为什么它没有得到对象的引用。

SeekBar xml文件：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">

<TextView
    android:id="@+id/seek_bar_title"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_alignParentStart="true"
    android:text="Alarm Volume"
    android:textColor="@android:color/black"
    android:textSize="16sp"
    android:layout_marginStart="16dp"
    android:layout_marginTop="16dp"/>

<TextView
    android:id="@+id/seek_bar_value"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_alignParentEnd="true"
    android:text="11"
    android:textColor="@android:color/black"
    android:textSize="16sp"
    android:layout_marginEnd="24dp"
    android:layout_marginTop="16dp"/>

<SeekBar
    android:id="@+id/alarm_volume_seek_bar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginTop="16dp"
    android:layout_marginBottom="16dp"
    android:layout_marginStart="8dp"
    android:layout_marginEnd="8dp"
    android:layout_below="@id/seek_bar_title"
    android:max="15"/>

偏好屏幕：

<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">

<PreferenceCategory
    android:title="SETTINGS">

    <Preference
        android:layout="@layout/seekbar_preference" />

    <CheckBoxPreference
        android:title="Countdown Notification"
        android:summary="show timers in notification tray"
        android:key="countdown_notification"
        android:defaultValue="true" />

    <CheckBoxPreference
        android:title="Keep Screen On"
        android:summary="keep screen on and app in foreground"
        android:key="keep_screen_on"
        android:defaultValue="false" />

</PreferenceCategory>

偏好活动：

public class AppPreferenceActivity extends PreferenceActivity {

TextView seekBarValue;
AudioManager audioManager = null;
SeekBar alarmVolumeSeekBar = null;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    getFragmentManager().beginTransaction()
            .replace(android.R.id.content, new SettingsFragment())
            .commit();

    initVolumeControl();
}

public static class SettingsFragment extends PreferenceFragment {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        addPreferencesFromResource(R.xml.preferences);
    }
}

public void initVolumeControl() {

    try {
        alarmVolumeSeekBar = (SeekBar) findViewById(R.id.alarm_volume_seek_bar);
        audioManager = (AudioManager) getSystemService(Context.AUDIO_SERVICE);
        seekBarValue = (TextView) findViewById(R.id.seek_bar_value);

        int alarmVolumeMax = audioManager.getStreamMaxVolume(AudioManager.STREAM_MUSIC);
        int progress = audioManager.getStreamVolume(AudioManager.STREAM_MUSIC);

        alarmVolumeSeekBar.setMax(alarmVolumeMax);
        alarmVolumeSeekBar.setProgress(progress);

        alarmVolumeSeekBar.setOnSeekBarChangeListener(new OnSeekBarChangeListener() {

            @Override
            public void onStartTrackingTouch(SeekBar seekBar) {

            }

            @Override
            public void onStopTrackingTouch(SeekBar seekBar) {

            }

            @Override
            public void onProgressChanged(SeekBar bar, int progress, boolean fromUser) {
                audioManager.setStreamVolume(AudioManager.STREAM_MUSIC, progress, 0);
                alarmVolumeSeekBar.setProgress(progress);
                seekBarValue.setText(progress);
            }
        });
    }
    catch (Exception e) {
        e.printStackTrace();
    }
  }
}