NullPointerException in FragmentManager

Question

I'm using the Android compatibility library and occasionally get a weird NullPointerException:

java.lang.NullPointerException
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:960)
    at android.support.v4.app.FragmentManagerImpl.performPendingDeferredStart(FragmentManager.java:768)
    at android.support.v4.app.FragmentManagerImpl.startPendingDeferredFragments(FragmentManager.java:1104)
    at android.support.v4.app.LoaderManagerImpl$LoaderInfo.onLoadComplete(LoaderManager.java:410)
    at android.support.v4.content.Loader.deliverResult(Loader.java:103)
    at android.support.v4.content.CursorLoader.deliverResult(CursorLoader.java:81)
    at android.support.v4.content.CursorLoader.onStartLoading(CursorLoader.java:126)
    at android.support.v4.content.Loader.startLoading(Loader.java:197)
    at android.support.v4.app.LoaderManagerImpl$LoaderInfo.start(LoaderManager.java:262)
    at android.support.v4.app.LoaderManagerImpl.doStart(LoaderManager.java:710)
    at android.support.v4.app.Fragment.onStart(Fragment.java:981)
    at android.support.v4.app.Fragment.performStart(Fragment.java:1332)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:906)
    at android.support.v4.app.FragmentManagerImpl.attachFragment(FragmentManager.java:1240)
    at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:612)
    at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1416)
    at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:431)
    at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:139)
    at android.support.v4.view.ViewPager.populate(ViewPager.java:804)
    at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:433)
    at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:405)
    at android.support.v4.view.ViewPager.setCurrentItem(ViewPager.java:386)
    ...

Obviously, I'm doing something wrong to allow the FragmentManager to get into such a state where it can crash like this, but I have no clue what. The relevant code in the FragmentManagerImpl is not offering me any clues. I'm guessing mActivity is somehow null at that point in the code? But that seems impossible as the activity is already on the screen and I'm not adding any fragments to it — just switching among them in a ViewPager.

Answer 1

I was getting the same error because I was referencing an instance of a fragment in a view pager adapter rather that the fragment itself.

Creating a new instance of the fragment each time the fragment was called resolved this error. Obviously this is proper programming but just in case some other noobs are making that mistake....

@Override
public Fragment getItem(int position) {
    if(position == 0)
        return new Walkthrough_Fragment_1();
    if(position == 1)
        return new Walkthrough_Fragment_4();
    if(position == 2)
        return new Walkthrough_Fragment_2();
    else{
        return new  Walkthrough_Fragment_3();
    }
}