Attempt to invoke virtual method 'android.os.Handler android.support.v4.app.FragmentHostCallback.getHandler()' on a null object reference

Question

My Application consist of 4 fragment as tabs being loaded inside a parent Fragment using FragmentPagerAdapter.

The problem is when I run the app and press back and re-open the app I receive this error log:

FATAL EXCEPTION: main
java.lang.NullPointerException: Attempt to invoke virtual method 'android.os.Handler android.support.v4.app.FragmentHostCallback.getHandler()' on a null object reference
    at android.support.v4.app.FragmentManagerImpl.ensureExecReady(FragmentManager.java:1949)
    at android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:1965)
    at android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:620)
    at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:143)
    at android.support.v4.view.ViewPager.setAdapter(ViewPager.java:513)
    ...

the line of code inside parent Fragment is:

viewPager.setAdapter(adapter);
tabLayout.setupWithViewPager(viewPager);

and ViewPage and Adapter both are not null!!

I have to mention that all my Fragments lifecycle is being managed and the null issue is happening inside the adapter!, and the same Adapter is working fine when I use an Activity as parent instead of Fragment!!!

Answer 1

After a lot of research, it looks like it is a support library issue:

Activity with fragment and ViewPager crashesh on configuration change
Android Support Library issue 24.1.0

I also found this answer, but it looks unusual adding a flag to my code:

Attempt to invoke virtual method ‘android.os.Handler android.support.v4.app.FragmentHostCallback.getHandler()’ on a null object

I override the finishUpdate method of my FragmentPagerAdapter like this and problem solved for now!

@Override
public void finishUpdate(ViewGroup container) {
    try{
        super.finishUpdate(container);
    } catch (NullPointerException nullPointerException){
        System.out.println("Catch the NullPointerException in FragmentPagerAdapter.finishUpdate");
    }
}

any better answer? please let me know...

Answer 2

I have the same issue on my project. But it is my fault.

WHY

I call onBackPressed in a callback after the activity has invoked onDestroy();

Because the mHost instance in the FragmentManager is set to null after onDestroy() called.

and when call onBackPressed it will use the mHost instance again.

Answer 3

@Override
public void onBackPressed() {
//     super.onBackPressed();
       finish();
}  

Override the onBackPressed in activity..
remove the super call : super.onBackPressed
and call finish.