经过数小时的搜索解决方案后，无法将按钮添加到操作栏

Question

经过几个小时寻找解决方案以查找代码中的错误，我最终在这里提出了一个问题。我尝试了所有提供的解决方案，没错。也许我的代码有问题。

因此，这些都是我的代码：

AboutUsActivity.java：

public class AboutUsActivity extends AppCompatActivity { 


@Override 
public boolean onOptionsItemSelected(MenuItem item) { 
    switch (item.getItemId()) { 
     // Respond to the action bar's Up/Home button 
     case android.R.id.home: 
      NavUtils.navigateUpFromSameTask(this); 
      return true; 
    } 
    return super.onOptionsItemSelected(item); 
} 

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

    getSupportActionBar().setDisplayHomeAsUpEnabled(true); 


    } 
} 

我AndroidManifest文件：

<application 
    android:allowBackup="true" 
    android:icon="@mipmap/ic_launcher" 
    android:label="@string/app_name" 
    android:roundIcon="@mipmap/ic_launcher_round" 
    android:supportsRtl="true" 
    android:theme="@style/AppTheme"> 
    <activity android:name=".activity.LoginActivity"> 
     <intent-filter> 
      <action android:name="android.intent.action.MAIN" /> 

      <category android:name="android.intent.category.LAUNCHER" /> 
     </intent-filter> 
    </activity> 
    <activity android:name=".activity.GeneralSettingActivity" android:parentActivityName=".activity.ProfileActivity"/> 
    <activity android:name=".activity.RegisterActivity" android:parentActivityName=".activity.LoginActivity"/> 
    <activity android:name=".activity.ResetPasswordActivity" android:parentActivityName=".activity.LoginActivity"/> 
    <activity android:name=".activity.HomeActivity" /> 
    <activity android:name=".activity.AboutUsActivity" 
     android:parentActivityName=".activity.HomeActivity" 
     android:theme="@style/AppTheme.ActionBar" 
     /> 
    <activity android:name=".activity.ApplyTutor" android:parentActivityName=".activity.HomeActivity"/> 
    <activity android:name=".activity.PrivacyPolicyActivity" android:parentActivityName=".activity.HomeActivity"/> 
    <activity 
     android:name=".activity.ProfileActivity" 
     android:label="@string/title_activity_profile" 
     android:theme="@style/AppTheme.NoActionBar" /> 
    <activity android:name=".activity.EditProfileActivity" android:parentActivityName=".activity.ProfileActivity"/> 
    <activity android:name=".activity.EarningActivity" android:parentActivityName=".activity.ProfileActivity"/> 
    <activity android:name=".activity.ScheduleActivity" android:parentActivityName=".activity.ProfileActivity"/> 
    <activity android:name=".activity.RatingReviewActivity" android:parentActivityName=".activity.ProfileActivity"/> 
    <activity android:name=".activity.RequestActivity"></activity> 
</application> 

我styles.xml：

<resources> 

<!-- Base application theme. --> 
<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar"> 
    <!-- Customize your theme here. --> 
    <item name="colorPrimary">@color/colorPrimary</item> 
    <item name="colorPrimaryDark">@color/colorPrimaryDark</item> 
    <item name="colorAccent">@color/colorAccent</item> 
    <item name="android:windowFullscreen">false</item> 
    <item name="windowActionBar">false</item> 

</style> 

<style name="AppTheme.NoActionBar"> 
    <item name="windowActionBar">false</item> 
    <item name="windowNoTitle">true</item> 

</style> 

<style name="AppTheme.ActionBar"/> 


<style name="AppTheme.AppBarOverlay" parent="ThemeOverlay.AppCompat.Dark.ActionBar" /> 
<style name="AppTheme.PopupOverlay" parent="ThemeOverlay.AppCompat.Light" /> 

我activity_about_us.xml：

<?xml version="1.0" encoding="utf-8"?> 
<android.support.constraint.ConstraintLayout 
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:layout_width="match_parent" 
android:layout_height="match_parent" 
tools:context="com.example.cqaai.tutorpal.activity.AboutUsActivity"> 

<TextView 
    android:layout_width="wrap_content" 
    android:layout_height="match_parent" 
    android:text="Page name: About Us"/> 

</android.support.constraint.ConstraintLayout> 

Android的显示器显示错误：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference 
                      at com.example.cqaai.tutorpal.activity.AboutUsActivity.onCreate(AboutUsActivity.java:32)