title: Back Navigation in Action Bar Not working
body: <p>I try to make back button in Action bar using this source. But it doesn't working.
  Here's the source.</p>
  
  <pre><code>protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_location_found);
          ActionBar actionBar = getActionBar();
          actionBar.setDisplayHomeAsUpEnabled(true);
      }
  </code></pre>
  
  <p>This code showing no error, but when I test it on emulator, it force close.
  Anyone know what's wrong?</p>
  
  <p>Here's the Log Chat:</p>
  
  <pre><code>12-23 10:49:02.725: D/dalvikvm(693): GC_CONCURRENT freed 190K, 5% free 7100K/7431K, paused 4ms+105ms
  12-23 10:49:33.345: D/AndroidRuntime(693): Shutting down VM
  12-23 10:49:33.355: W/dalvikvm(693): threadid=1: thread exiting with uncaught exception (group=0x409961f8)
  12-23 10:49:33.405: E/AndroidRuntime(693): FATAL EXCEPTION: main
  12-23 10:49:33.405: E/AndroidRuntime(693): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.okeschool/com.okeschool.ajax}: java.lang.NullPointerException
  12-23 10:49:33.405: E/AndroidRuntime(693):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1955)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at android.app.ActivityThread.access$600(ActivityThread.java:122)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at android.os.Handler.dispatchMessage(Handler.java:99)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at android.os.Looper.loop(Looper.java:137)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at android.app.ActivityThread.main(ActivityThread.java:4340)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at java.lang.reflect.Method.invokeNative(Native Method)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at java.lang.reflect.Method.invoke(Method.java:511)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at dalvik.system.NativeStart.main(Native Method)
  12-23 10:49:33.405: E/AndroidRuntime(693): Caused by: java.lang.NullPointerException
  12-23 10:49:33.405: E/AndroidRuntime(693):  at com.okeschool.ajax.onCreate(ajax.java:15)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at android.app.Activity.performCreate(Activity.java:4465)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
  12-23 10:49:33.405: E/AndroidRuntime(693):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1919)
  12-23 10:49:33.405: E/AndroidRuntime(693):  ... 11 more
  </code></pre>