How to get hashmap response using Retrofit 2

I am trying to fetch json object response using retrofit 2. I used hashmap as the keys are dynamic. Here is my response class :

public class Countries {

    private Map<String, Model> datas;

    public Map<String, Model> getDatas() {
        return datas;
    }
}

And the Model class is :

public class Model {

    @SerializedName("country_name")
    private String country_name;
    @SerializedName("continent_name")
    private String continent_name;

    public String getCountry_name() {
        return country_name;
    }

    public String getContinent_name() {
        return continent_name;
    }
}

So far, i've tried to handle the response like this :

call.enqueue(new Callback<Countries>() {
            @Override
            public void onResponse(Call<Countries> call, Response<Countries> response) {

                Map<String, Model> map = new HashMap<String, Model>();
                map = response.body().getDatas();

                for (String keys: map.keySet()) {
                    // myCode;
                }

            }

            @Override
            public void onFailure(Call<Countries> call, Throwable t) {

            }
        });

And this error occurs :

java.lang.NullPointerException: Attempt to invoke interface method 'java.util.Set java.util.Map.keySet()' on a null object reference

JSON response looks like this :

{
    "0": {
        "country_name": "Argentina",
        "continent_name": "South America"
    },
    "1": {
        "country_name": "Germany",
        "continent_name": "Europe"
    }
}

So how can I get the response in HashMap?