Android getSupportActionBar（）返回null

我正在使用

支持操作栏

。它在

MessageTemplateActivity

中工作，因为当我想将enable显示为

getSupportActionBar（）。setDisplayHomeAsUpEnabled（true），它正在启用“后退”按钮。但是在
SyncActivity
中，
getSupportActionBar（）返回
null
。有人能告诉我为什么在
SyncActivity`中返回空值吗

空指针异常
布局\同步\活动.xml
layout.layout_message_template.xml


谢谢

06-17 18:45:45.327: E/AndroidRuntime(25335): Caused by: java.lang.NullPointerException
06-17 18:45:45.327: E/AndroidRuntime(25335):    at android.support.v7.app.ActionBarImplICS.getDisplayOptions(ActionBarImplICS.java:234)
06-17 18:45:45.327: E/AndroidRuntime(25335):    at android.support.v7.app.ActionBarImplICS.<init>(ActionBarImplICS.java:57)
06-17 18:45:45.327: E/AndroidRuntime(25335):    at android.support.v7.app.ActionBarImplICS.<init>(ActionBarImplICS.java:46)
06-17 18:45:45.327: E/AndroidRuntime(25335):    at android.support.v7.app.ActionBarActivityDelegateICS.createSupportActionBar(ActionBarActivityDelegateICS.java:46)
06-17 18:45:45.327: E/AndroidRuntime(25335):    at android.support.v7.app.ActionBarActivityDelegate.getSupportActionBar(ActionBarActivityDelegate.java:83)
06-17 18:45:45.327: E/AndroidRuntime(25335):    at android.support.v7.app.ActionBarActivity.getSupportActionBar(ActionBarActivity.java:66)

public class SyncActivity extends ActionBarActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.layout_sync_activity);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    }
}

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/mainSyncScrollView"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
</RelativeLayout>

public class MessageTemplateActivity extends ActionBarActivity{
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.layout_message_template);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    }

}

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/mainScrollView"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
</ScrollView>