throw nullpointer exception even pass the requirement , any ideas ? thanks!

ArrayList<String> listUpperCase = new ArrayList<String>();

        for (int i = 0; i < list.size(); i++) {
            String s = list.get(i);
            int len = s.length();
            if(len%2==0){
            listUpperCase.add(s+" "+s);
            }
            else if((len%2!=0)){
            listUpperCase.add(s+" "+s+" "+s);
            }
        }