Add an object to an arraylist using reflection

I have this setup

public class ClassA {   
    //I cannot modify or access this class
    private List<Object> list = new ArrayList<>();  
}

public class ClassB {

    public void entry(String... args) {
        ClassA instance = ClassRegistry.find(ClassA.class); 
        //add using reflection
        //instance.list.add(new Object());
    }
}

how do I add an object to that list using reflection?

I tried

Field list = instance.getClass().getDeclaredField("list");
list.setAccessible(true);
ArrayList<Object> actualList = (ArrayList<Object>) list.get(instance);
actualList.add(new Object());
list.set(instance, actualList);

but it didn't work. the list was still empty.