findfragmentbyid 返回 null
FragmentTransaction transaction = getFragmentManager().beginTransaction();
MyFragment fragment = new MyFragment();
transaction.add(android.R.id.content, fragment,"tag").addToBackStack(null).commit();
Log.d("POPUP", getFragmentManager().findFragmentById(android.R.id.content) + "");
Log.d("POPUP", getFragmentManager().findFragmentByTag("tag") + "");