Подскажите в чем проблема, компилятор не пропускает

public static void main(String[] args) {
        ArrayList<String> strings = new ArrayList<String>();
        strings.add("роза");
        strings.add("лоза");
        strings.add("лира");
        strings = fix(strings);

        for (String string : strings) {
            System.out.println(string);
        }
    }

    public static ArrayList<String> fix(ArrayList<String> strings) {

        for (int i = 0; i < strings.size(); i++)
        {
            String name = strings.get(i);
            if (strings.contains("р") || strings.contains("л"))
            {

        }
            else if (strings.contains("р"));
            {
                strings.remove(name);
        }
            if  (strings.contains("л"))
            {
                strings.add(name);
                strings.add(name);
        }
            else {

            }
        }


        return null;
    }
}

C:\Users\ilyac\.jdks\openjdk-15.0.2\bin\java.exe "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.3\lib\idea_rt.jar=60608:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.3\bin" -Dfile.encoding=UTF-8 -classpath C:\Users\ilyac\IdeaProjects\JavaRush\out\production\JavaRush JavaWork.Solution
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.util.ArrayList.iterator()" because "strings" is null
	at JavaWork.Solution.main(Solution.java:19)

Process finished with exit code 1