Android: Error in getcontext() when using Sharedpreferences

Question

I’m using SharedPreferences with Retrofit2 and notificações. Each time the Notification is executed I get the exception "'java.lang.String android.content.Context.getPackageName()' on a null object reference".

I’m calling the Retrofit to execute the notification of a class that extend of BroadcastReceiver:

public class MyReceiver extends BroadcastReceiver {

public void createNotification(Context context, String exemplo) {

        ExtendedActivity ext = new ExtendedActivity();
        ext.start(exemplo);
(...)

Class Extendedactivity

public class ExtendedActivity extends BaseActivity implements Callback<Channel>{

String descript="descript";

@Override
    public void start(String texto) {
    (...)
}

@Override
    public void onResponse(Call<Channel> call, Response<Channel> response) {
    if (response.isSuccessful()) {
        Channel rss = response.body();

        saveData(descript, rss.getItemDescription()); <--------- ERRO
     }
 }

    public void saveData(String key, String time){
        SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this.getContext()); <-----ERRO
        SharedPreferences.Editor editor = preferences.edit();

        if (preferences.contains(key)) {
            editor.remove(key);
        }

    editor.putString(key, time);
    editor.commit();
}

The problem is in the context of the method saveData. I have tried to use the context of other activities and it keeps giving me error. Someone can help?