docs: public void removeLocation(final String key, final CompletionListener completionListener) {
          if (key == null) {
              throw new NullPointerException();
          }
          DatabaseReference keyRef = this.getDatabaseRefForKey(key);
          if (completionListener != null) {
              keyRef.setValue(null, new DatabaseReference.CompletionListener() {
                  @Override
                  public void onComplete(DatabaseError databaseError, DatabaseReference databaseReference) {
                      completionListener.onComplete(key, databaseError);
                  }
              });
          } else {
              keyRef.setValue(null);
          }
      }