Answering my own question:
This exception is (eventually) thrown when you call
EDIT: And also, like Twice Circled and shinyuX point out in the comment; when calling the
detach(null) methods, and probably also
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
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
null before removing the fragment.