Android FragmentManager throwing NullPointerException

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.

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