NullPointerException when calling cursor in a method from another class

Good, I have the following problem or doubt is when executing the following method in my activity main

public String ObtenerDatos(String persona) {

HashMap<String,String> numero = new HashMap<String,String>();
String[] projeccion = new String[]{ContactsContract.Data.DISPLAY_NAME, ContactsContract.CommonDataKinds.Phone.NUMBER};
String selectionClause = ContactsContract.Data.MIMETYPE + "='" +
        ContactsContract.CommonDataKinds.Phone.CONTENT_ITEM_TYPE + "' AND "
        + ContactsContract.CommonDataKinds.Phone.NUMBER + " IS NOT NULL";
String sortOrder = ContactsContract.Data.DISPLAY_NAME + " ASC";
Cursor c = getContentResolver().query(
        ContactsContract.Data.CONTENT_URI,
        projeccion,
        selectionClause,
        null,
        sortOrder);

numero.clear();
while (c.moveToNext()) {
    numero.put(c.getString(0).toLowerCase() , c.getString(1));

}
c.close();
Iterator it = numero.keySet().iterator();
String select = null;
select = numero.get(persona);
if (select == null || select == ""){
    select = "123";
}
return select;}

this works without problems, but when calling the same method from another class

Contactos contact = new Contactos();
contact.ObtenerDatos("casa");

I generate a NullPointerException, for which I do not know how to solve this problem I appreciate your reading and possible clarifications.

I add: this error occurs when in class Contactos in method ObtenerDatos when% is called cursor c