Android FragmentManager BackStackRecord.run throwing NullPointerException

Question

I sometimes get the following exception when working with Fragments:

FATAL EXCEPTION: main
java.lang.NullPointerExce         


        

Answer 1

Answering my own question:

This exception is (eventually) thrown when you call FragmentTransaction.remove(null); and FragmentTransaction.commit();

EDIT: And also, like Twice Circled and shinyuX point out in the comment; when calling the show(null) or add(null), attach(null) and detach(null) methods, and probably also hide(null)

After calling commit(), the transaction will be queued in the FragmentManager. As a result, when the operation is being processed after you explicitly call FragmentManager.executePendingTransactions(), or when the FragmentManager queue thread calls it, it throws a NullPointerException.

In my case, I was maintaining fragment states in a global object. There I checked if the fragment was showing or not, and then removed visible fragments. But because I started a new FragmentActivity, these states were still set to true while they were not visible. So this is a design error.

Other than fixing the design error, the solution was simple: check whether FragmentManager.findFragmentByTag() returned null before removing the fragment.

Answer 2

The one reason why it happens it is invoking

getSupportFragmentManager().beginTransaction().remove(fragment)

while fragment is null

Answer 3

I don't use tag to create the fragments (they works like TabBar containers).

So, it works when change Tab, but if I press back button I got the same error.

At onDestroyView method I found fragment instance with FragmentManager#findFragmentById, however FragmentManager#findFragmentByTag returns null, sure.

class MyFragment extends ListFragment {

@Override
public void onDestroyView() {
    super.onDestroyView();

    if (this.mapFragment != null
            && getFragmentManager().findFragmentById(
                    this.mapFragment.getId()) != null) {

        getFragmentManager().beginTransaction().remove(this.mapFragment)
                .commit();
        this.mapFragment = null;
    }

}
}