Android listview checkbox

I have a listbox where i choose the elements i want to download by a checkbox, and a FloatingActionButton to confirm. The problem is that on my phone(Android 5.0.2) i get a "Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference" On my tablet (Nexus7 Android 6.0) everything work!

        FloatingActionButtonDone.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            for (int j = 0; j < ListActivityListView.getCount(); j++) {

                v = ListActivityListView.getChildAt(j);

                //Here im having troubles!
                ck = (CheckBox) v.findViewById(R.id.ListActivityCheckBox);


                if (ck.isChecked()) {

                    /*
                    Stuff to download ++
                    */

                }

            }
            /*
            Here I have the code to download my stuff
            */
        }//End of onClick

    });//End of  FloatingActionButtonDone.setOnClickListener

Where: - ck is a CheckBox ck, declared in my class variables. - ListActivityListView is the ListView inside my activity.

I repeat same code works on my table, but doesn't on my phone. So i have literally no idea. Since my project is composed by 12 classes, i tried to post here the relevant code i'm having trouble with.