Do an assertion here to get rid of a warning - this can not be null

src/main/java/org/ligi/passandroid/model/PastLocationsStore.java

@@ -30,6 +30,7 @@ public class PastLocationsStore {
         final SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);
         final Set<String> pastLocations = prefs.getStringSet(KEY_PAST_LOCATIONS, new HashSet<String>());
 
+        assert pastLocations != null;
         if (pastLocations.size() >= MAX_ELEMENTS) {
             deleteOneElementFromSet(pastLocations);
         }