Android : How to the fixed NullPointerException?

I used navigationdawerfragment. step by step application excute -> Home button click -> idle 5 minute -> application excute -> died public void restoreActionBar() {

ActionBar actionBar = getSupportActionBar(); Log.e("MainActivity", "actionBar = "+ actionBar); actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD); actionBar.setDisplayShowTitleEnabled(true); actionBar.setTitle(mTitle); } why is the null value of the actionbar? memory problem?? Ask Question. please.

09-25 13:59:40.459: E/AndroidRuntime(25192): FATAL EXCEPTION: main 
    09-25 13:59:40.459: E/AndroidRuntime(25192): Process: com.donut.todayshop, PID: 25192 
    09-25 13:59:40.459: E/AndroidRuntime(25192): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.donut.todayshop/com.donut.todayshop.MainActivity}: java.lang.NullPointerException 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2413) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2471) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.app.ActivityThread.access$900(ActivityThread.java:175) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.os.Handler.dispatchMessage(Handler.java:102) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.os.Looper.loop(Looper.java:146) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.app.ActivityThread.main(ActivityThread.java:5602) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at java.lang.reflect.Method.invokeNative(Native Method) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at java.lang.reflect.Method.invoke(Method.java:515) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at dalvik.system.NativeSt`enter code here`art.main(Native Method) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): Caused by: java.lang.NullPointerException 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at com.donut.todayshop.MainActivity.restoreActionBar(MainActivity.java:156) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at com.donut.todayshop.ImageFragment.onAttach(ImageFragment.java:89) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:898) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1121) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1103) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.support.v4.app.FragmentManagerImpl.dispatchCreate(FragmentManager.java:1896) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:216) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.support.v7.app.ActionBarActivity.onCreate(ActionBarActivity.java:98) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at com.donut.todayshop.MainActivity.onCreate(MainActivity.java:60) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.app.Activity.performCreate(Activity.java:5451) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1093) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2377) 
    09-25 13:59:40.459: E/AndroidRuntime(25192): ... 11 more