NullPointerException: android.support.v4.app.FragmentHostCallback.getHandler() on a null object reference

Question

I am facing following issue.

Scenario:

For the very first time when I signup thru app, I am able to see tabs and there content but when I logout and try to login for the same code for viewpager i get following exception:

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

but when open navigation menu and select after that I get data and tabs are shown viewpager shows tabs.

Contents: I have 2 tabs

Below is my code:

 public void setupDashboardTabs(List<CreateFragmentsPojo> fragments) {
    this.fragments = fragments;

    viewPager = (ViewPager) findViewById(R.id.viewpager); 
    viewPager.setOffscreenPageLimit(3);

    setupDashboardViewPager(fragments);

    tabLayout = (TabLayout) findViewById(R.id.tabs);
    tabLayout.setupWithViewPager(viewPager);

    setUpFontStyleForTabs();
}


public void setupDashboardViewPager(List<CreateFragmentsPojo> fragments) {
    try {
        ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());

        for (int i = 0; i < fragments.size(); i++) {
            adapter.addFragment(fragments.get(i).getFragment(), fragments.get(i).getTitle());
        }
        viewPager.setAdapter(adapter);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

viewPager.setAdapter(adapter); getting nullpointer execption at this line

Code to logout:

 Intent intent = new Intent(this, LoginActivity.class);
    intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_NEW_TASK);
    startActivity(intent);
    finish();

Stacktrace:

java.lang.NullPointerException: Attempt to invoke virtual method 'android.os.Handler android.support.v4.app.FragmentHostCallback.getHandler()' on a null object reference
 W/System.err:     at android.support.v4.app.FragmentManagerImpl.ensureExecReady(FragmentManager.java:1949)
 W/System.err:     at android.support.v4.app.FragmentManagerImpl.execSingleAction(FragmentManager.java:1965)
 W/System.err:     at android.support.v4.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:620)
 W/System.err:     at android.support.v4.app.FragmentStatePagerAdapter.finishUpdate(FragmentStatePagerAdapter.java:166)
 W/System.err:     at android.support.v4.view.ViewPager.setAdapter(ViewPager.java:513)
 W/System.err:     at io.skreem.dashboard.DashboardActivity.setupDashboardViewPager(DashboardActivity.java:459)
 W/System.err:     at io.skreem.dashboard.DashboardActivity.setupDashboardTabs(DashboardActivity.java:443)
 W/System.err:     at io.skreem.dashboard.DashboardActivity$4.onResponse(DashboardActivity.java:408)
 W/System.err:     at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.run(ExecutorCallAdapterFactory.java:68)
 W/System.err:     at android.os.Handler.handleCallback(Handler.java:746)
 W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:95)
 W/System.err:     at android.os.Looper.loop(Looper.java:148)
 W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:5443)
 W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
 W/System.err:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
 W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)

Answer 1

viewPager.setOffscreenPageLimit(3);

is the problem. As you have 2 tabs. Which means 2 pages but you are forcing 3 as the PageLimit. Also whats the need of adding fragment to adapter?

Answer 2

Remove the super.onBackPressed() and call finish()

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

I don't know why but this worked in my case!

Answer 3

I had same issue and it has been caused by the reuse of ViewPager instance by different instances of the fragment. There was an exception after ViewPager.setAdapter() has been called for the second time. If this is the issue you experience, you can try a solution I used. To solve the issue I had to remove adapter in onDestroyView() method of my fragment.

Project demoing the issue and the solution can be found here.