After a while searching the reason of my NPE, i noticed that despite FragmentTransaction.remove(Fragment) reallys removes the Fragment, the size of the List returned by FragmentManager.getFragments() still the same. This screenshot shows what is returned by this method after a fragment removal:
https://www.dropbox.com/s/s3bt1p7i71a5kgx/erro%20fm.PNG
The list have 3 fragments, why size still 4?