public class Dog extends RealmObject {
    private String name;
    private int age;

    // ... Generated getters and setters ...
}

RealmList<Dog > dogs= new RealmList<>();
dogs.add(null);

 Caused by: java.lang.IllegalArgumentException: RealmList does not accept null values
                                                                 at io.realm.RealmList.checkValidObject(RealmList.java:733)
                                                                 at io.realm.RealmList.add(RealmList.java:187)