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

Ok guys, after hitting my head against a brick wall for a while on this I found that this was directly associated with my fragment's declaration of setRetainInstance(true). After removing this the problems went away. This appears to be a compatibility library bug...

I will raise something under the appropriate Google project. Best of luck if you are reading this slowly sobbering to yourself! I hope this will allow you to work around the problem.

Answer 2

I got this error a few days ago and was quite confounded, but I figured out that it was because a FragmentTransaction didn't seem to get any pending transactions so when calling executePendingTransactions(); A NPE much like this one was thrown.
I solved it by making sure that every transaction actually changed something (i.e add fragment 1 -> remove fragment 1 -> commit/execute would not work).
Also make sure that none of the fragments in the pager have become null.